Twins Glossary: Plain-Language Definitions for Twin Families

A rare condition in which one individual carries two genetically distinct sets of cells — sometimes arising from the fusion of two embryos.

In twins, chimerism can occur when two separately fertilized eggs fuse very early in development, producing a single individual who carries cells with two distinct sets of chromosomes. Some chimeric individuals are identified by unusual findings such as multiple blood types or cells that are simultaneously XX and XY. Chimerism may also arise from fused placentas in fraternal twin pregnancies, allowing the circulatory systems to intermix. Fewer than 25 confirmed human cases are documented in medical literature, though many cases are likely never detected. The term comes from the Chimera of Greek mythology — a creature composed of parts from different animals.

Identical twins whose bodies are physically connected at birth.

Conjoined twins result from an incomplete split of a monozygotic embryo, occurring very late in development (after day 13). They are physically joined at one or more points on the body. Conjoined twinning is extremely rare, occurring in roughly 1 in 50,000–60,000 births. The degree of connection and shared organs varies significantly.

The technical term for fraternal twins — formed from two separately fertilized eggs.

Dizygotic refers to the origin of fraternal twins: two separate eggs each fertilized by a different sperm, resulting in two genetically distinct embryos. Dizygotic twins share approximately 50% of their DNA — the same as any full siblings — and can be the same sex or different sexes. All dizygotic twins are dichorionic, meaning each baby has its own placenta. Dizygotic twinning is influenced by genetics, maternal age, height, and fertility treatments. See also: fraternal twins.

Fraternal twins — formed from two separate eggs fertilized by two separate sperm.

Dizygotic (DZ) twins develop from two separate eggs each fertilized by a different sperm. They share approximately 50% of their DNA — the same as any siblings — and can be the same sex or different sexes. Dizygotic twinning is influenced by genetics, maternal age, and fertility treatments. Also called fraternal twins.

Twins from two separate eggs — the most common type.

The common term for dizygotic twins. Fraternal twins are no more genetically similar than any other siblings, share approximately 50% of their DNA, and can be any combination of sexes. They are the most common type of twins overall and the type most influenced by fertility treatments such as IVF.

A rare phenomenon in which fraternal twins are conceived from two different biological fathers.

Heteropaternal superfecundation occurs when a woman releases more than one egg during a cycle and each egg is fertilized by sperm from a different partner. The result is fraternal twins who share a mother but have different biological fathers. This is only possible with fraternal (dizygotic) twins; identical twins originate from a single egg and cannot have different fathers. DNA testing has confirmed documented cases, including situations arising from IVF laboratory errors. It is distinct from superfecundation (same father, multiple eggs), which is the more common mechanism behind fraternal twinning.

Twins from a single egg that split — sharing nearly 100% of their DNA.

The common term for monozygotic twins. Despite sharing almost identical DNA, identical twins are distinct individuals — differences in gene expression, environment, and life experience mean they develop unique personalities, health histories, and sometimes even appearances over time.

Identical twins whose features appear as reflections of each other.

A subset of monozygotic twins in which the split occurs late — around day 9 to 12 after fertilization. Mirror twins may display reversed asymmetric features: one twin may be right-handed where the other is left-handed, or have birthmarks, hair whorls, or other features appearing on opposite sides. Not a separate biological type, but a recognized pattern within identical twinning.

The technical term for identical twins — formed when a single fertilized egg splits into two.

Monozygotic refers to the origin of identical twins: a single fertilized egg (zygote) that splits into two embryos. Monozygotic twins share nearly 100% of their DNA and are always the same biological sex. The timing of the split determines chorionicity and amnionicity — earlier splits result in dichorionic twins (each with their own placenta), while later splits produce monochorionic twins who share a placenta and face a different set of clinical risks. Small genetic and epigenetic differences can develop over time. See also: identical twins.

Identical twins — formed when a single fertilized egg splits into two.

Monozygotic (MZ) twins develop when a single fertilized egg (zygote) splits into two embryos. They share nearly 100% of their DNA and are always the same sex. The timing of the split determines chorionicity and amnionicity — earlier splits are less risky than late ones. Also called identical twins, though small genetic and epigenetic differences can develop over time.

Twins formed when an egg and its polar body are both fertilized.

A theorized but rarely confirmed type of twinning in which the polar body — a cell produced during egg development that normally dies — is fertilized by a separate sperm alongside the main egg. Polar body twins would be genetically between identical and fraternal twins. Sometimes called half-identical twins.

A rare third twin type — genetically identical on the maternal side but sharing only about half their paternal DNA.

Sesquizygotic twins (from the Latin for “one and a half”) arise when a single egg is fertilized by two sperm simultaneously — a process called polyspermy. Rather than producing a non-viable triploid embryo as typically occurs, the extra genetic material is shed during early cell division, resulting in two viable embryos that share all of their mother’s DNA but each carry different paternal genes. Only two confirmed cases appear in the medical literature, the most recent published in the New England Journal of Medicine in 2019. Sesquizygotic twins are genetically more similar than fraternal twins but not fully identical.

An extremely rare phenomenon where a second pregnancy begins during an existing one.

Superfetation occurs when a second egg is fertilized and implants in a uterus that already contains a developing embryo. The resulting twins can have different gestational ages and sometimes different fathers (heteropaternal superfetation). Extremely rare in humans — fewer than a dozen confirmed cases exist in medical literature.

When one twin from a multiple pregnancy is lost, often early in the first trimester.

Vanishing twin syndrome occurs when one embryo in a multiple pregnancy stops developing and is reabsorbed by the mother's body or the surviving twin's placenta. It most commonly occurs in the first trimester and may go undetected unless an early ultrasound has confirmed two embryos. The surviving twin typically continues to develop normally. Some IVF pregnancies that begin as twins become singleton pregnancies this way.

Whether twins are identical (monozygotic) or fraternal (dizygotic).

Zygosity refers to the genetic relationship between twins — specifically whether they originated from one fertilized egg (monozygotic/identical) or two (dizygotic/fraternal). Zygosity cannot be reliably determined by appearance alone and is definitively established only through DNA testing. Knowing zygosity matters for health reasons, including understanding shared genetic disease risk.

DNA testing that definitively determines whether twins are identical or fraternal.

A simple cheek swab or blood test that compares specific DNA markers between twins to determine zygosity. Unlike placenta examination at birth — which is unreliable in many cases — DNA testing is definitive. Many families discover twins they assumed were fraternal are actually identical, or vice versa. Testing is available privately and is not routinely offered on the NHS or most public health systems.

A fertilized egg — the single cell formed when a sperm fertilizes an egg, before it begins dividing.

A zygote is the cell formed at the moment of fertilization, when a sperm cell successfully penetrates and fuses with an egg. It contains the full set of chromosomes — half from each parent — and is the starting point of all embryonic development. In twin pregnancies, a single zygote may split to form identical (monozygotic) twins, or two separate zygotes may develop from two fertilized eggs to form fraternal (dizygotic) twins.

A lab technique that creates a small opening in the embryo's outer shell to aid implantation.

Assisted hatching involves making a small breach in the zona pellucida — the outer shell of the embryo — using laser, chemical, or mechanical means, to help the embryo hatch and implant in the uterine lining. It is used in some IVF cases to improve implantation rates. Some researchers believe assisted hatching may increase the chance of embryo splitting, potentially contributing to the higher identical twin rate seen in IVF pregnancies.

An embryo that has developed for 5–6 days to the stage just before implantation.

A blastocyst is an embryo that has developed for approximately 5 to 6 days after fertilization, reaching a stage characterized by a fluid-filled cavity and distinct cell layers. Most IVF clinics now culture embryos to the blastocyst stage before transfer, as blastocysts have higher implantation success rates than earlier-stage embryos. The blastocyst stage is also when embryo splitting — leading to identical twins — most commonly occurs.

Hormone treatment used in IVF to encourage the ovaries to produce multiple eggs.

Controlled ovarian stimulation (COS) uses injectable hormone medications — typically FSH and LH — to stimulate the ovaries to develop multiple follicles and eggs in a single cycle. This maximizes the number of eggs available for fertilization. COS is a standard part of IVF and also increases the chance of fraternal twins in non-IVF fertility treatments if multiple eggs are released and fertilized naturally.

An embryo with the correct number of chromosomes — 46 in humans.

A euploid embryo has the normal complement of 46 chromosomes and is less likely to result in miscarriage or failed implantation than an aneuploid embryo. Confirmed through PGT-A testing. Transferring a single euploid embryo has success rates that approach or match transferring two untested embryos in many patient profiles, making it the foundation of the case for single embryo transfer.

An IVF cycle where a previously frozen embryo is thawed and transferred.

In a frozen embryo transfer, embryos created during a previous IVF cycle are thawed and transferred in a subsequent cycle. FET has become the dominant approach in many clinics as embryo freezing technology has improved. Notably, identical twinning rates may be slightly higher with FET than with fresh transfers, though the reasons are not fully understood.

A person who carries a pregnancy using an embryo that is not genetically their own.

A gestational carrier (sometimes called a gestational surrogate) carries a pregnancy created using the intended parents' embryo — meaning the carrier has no genetic connection to the child. Gestational carriers are used by same-sex male couples, individuals who cannot safely carry a pregnancy, and others. The carrier's health history, preferences, and risk tolerance are central to decisions about how many embryos to transfer.

An IVF technique where a single sperm is injected directly into an egg.

ICSI involves injecting a single sperm directly into a mature egg using a fine needle, rather than mixing sperm and eggs in a dish and allowing fertilization to occur naturally. It is used primarily in cases of male factor infertility. Some research suggests ICSI may be associated with a slightly higher rate of embryo splitting — and therefore identical twins — though the mechanism is debated.

A fertility procedure where sperm is placed directly in the uterus around the time of ovulation.

IUI involves placing washed, concentrated sperm directly into the uterus through a thin catheter, timed to coincide with ovulation. When combined with ovarian stimulation medications, IUI increases the chance of fraternal twins because multiple eggs may be released. IUI does not increase the rate of identical twins. It is generally less expensive and less invasive than IVF.

A fertility treatment where eggs are fertilized outside the body and then transferred to the uterus.

IVF involves stimulating the ovaries to produce multiple eggs, retrieving those eggs, fertilizing them with sperm in a laboratory, and then transferring one or more resulting embryos into the uterus. It is the most effective form of assisted reproductive technology (ART) and the treatment most associated with increased twin rates, both through multiple embryo transfer and the higher rate of spontaneous embryo splitting it appears to cause.

Genetic screening of embryos before transfer to check for chromosomal abnormalities.

PGT-A screens embryos created through IVF for aneuploidies — abnormal numbers of chromosomes. Embryos with the correct chromosome count are called euploid and have higher implantation rates and lower miscarriage rates. PGT-A testing has contributed to the shift toward single embryo transfer, since transferring one tested euploid embryo offers success rates comparable to transferring two untested embryos, with significantly lower twin risk.

An IVF arrangement where one partner provides the eggs and the other carries the pregnancy.

Reciprocal IVF — also called partner IVF or co-IVF — is used by same-sex female couples and some transgender individuals. One partner undergoes egg retrieval; the eggs are fertilized and the resulting embryo(s) are transferred to the other partner's uterus. Both partners have a biological role — one genetic, one gestational. If two embryos are transferred and both implant, the twins may each have a different biological connection to each parent.

Transferring one embryo at a time during IVF — the current best-practice recommendation for most patients.

Single embryo transfer is the practice of placing one embryo into the uterus per IVF cycle rather than two or more. Current guidelines from the American Society for Reproductive Medicine (ASRM) recommend SET for most patients under 38 with good-quality embryos. Research consistently shows SET does not significantly reduce overall live birth rates compared to transferring two embryos, but substantially reduces the risk of twin pregnancy and its associated complications.

Whether twins have separate amniotic sacs (diamniotic) or share one (monoamniotic).

Amnionicity describes whether each twin has its own amniotic sac (diamniotic) or whether both twins share a single sac (monoamniotic). Monoamniotic twins — who share both placenta and sac — are the highest-risk category of twin pregnancy due to the risk of cord entanglement. The vast majority of twin pregnancies are diamniotic. Amnionicity is assessed alongside chorionicity at the early ultrasound.

Weight in kilograms divided by height in metres squared.

Body mass index is a screening measure calculated from weight and height. In pregnancy, weight-gain guidance is usually set by pre-pregnancy BMI category (underweight, normal weight, overweight, or obese). It is a general guide, not a diagnosis.

The outermost fetal membrane — the structure that determines chorionicity in twins.

The chorion is the outermost of the membranes surrounding the developing embryo. It forms early in pregnancy and eventually develops into the fetal side of the placenta. In twin pregnancies, if both embryos are enclosed within a single chorion (monochorionic), they will share a placenta. If each embryo develops its own chorion (dichorionic), each will have a separate placenta. The chorion is the structure that determines chorionicity — the most clinically significant factor in a twin pregnancy.

Whether twins share a placenta (monochorionic) or have separate placentas (dichorionic).

Chorionicity describes the placental arrangement in a twin pregnancy — specifically whether the twins share one placenta (monochorionic) or each have their own (dichorionic). It is determined by ultrasound, ideally in the first trimester when it is most reliably assessed. Chorionicity is the single most important factor in determining the risk level and monitoring schedule for a twin pregnancy. All monochorionic twins are identical; dichorionic twins may be identical or fraternal.

A risk exclusive to monoamniotic twins, where umbilical cords can become intertwined in the shared sac.

Cord entanglement occurs when the umbilical cords of monoamniotic (Mo-Mo) twins become tangled within their shared amniotic sac. Because there is no separating membrane, the cords can wrap around each other and restrict blood flow to one or both babies. It is one of the primary reasons monoamniotic pregnancies require hospitalization from approximately 28 weeks and early delivery. Cord entanglement cannot occur in diamniotic pregnancies where a separating membrane is present.

Each twin has their own separate amniotic sac.

Diamniotic describes a twin pregnancy in which each baby is enclosed in its own amniotic sac, separated by a membrane. The vast majority of twin pregnancies — both dichorionic and most monochorionic — are diamniotic. Having separate sacs eliminates the risk of cord entanglement that exists in monoamniotic pregnancies. In monochorionic diamniotic (Mo-Di) pregnancies, twins share a placenta but have separate sacs.

Having two separate chorions (outer membranes) — meaning each twin has their own placenta.

Dichorionic describes a twin pregnancy in which each baby has developed its own chorion and its own placenta. All fraternal (dizygotic) twins are dichorionic. Roughly 25–30% of identical (monozygotic) twins are also dichorionic, having split very early — within the first three days after fertilization. Dichorionic twins carry a lower risk profile than monochorionic twins because they do not share a placental blood supply.

Twins with separate placentas and separate amniotic sacs — the lowest-risk twin arrangement.

Di-Di twins each have their own placenta and their own amniotic sac. They may be fraternal (most common) or identical if the embryo split very early — within the first three days after fertilization. Di-Di is the lowest-risk twin arrangement because the twins do not share a placenta, eliminating risks like TTTS. All fraternal twins are Di-Di.

A baby's weight calculated from ultrasound measurements of the head, belly and thigh bone.

Estimated fetal weight (EFW) is calculated by plugging ultrasound measurements (typically head size, abdominal circumference and femur length) into a standard formula such as Hadlock. Because it is an estimate, it can differ from the actual birth weight, so clinicians interpret it alongside growth over time and other scan findings.

A detailed ultrasound of the fetal heart — recommended in all monochorionic pregnancies.

A fetal echocardiogram is a specialized ultrasound examination of the developing fetal heart, assessing its structure, valves, and blood flow in detail. It is recommended between 18 and 22 weeks in monochorionic twin pregnancies because of an elevated risk — roughly 3.5 times higher odds ratio — of congenital heart defects compared to singleton or dichorionic pregnancies. It is typically performed by a maternal-fetal medicine specialist or a pediatric cardiologist with fetal imaging expertise.

A tape-measure check of uterus size — one of the first ways a provider might suspect twins.

Fundal height is the distance in centimeters from the pubic bone to the top of the uterus (the fundus), measured during prenatal appointments. In a singleton pregnancy, it roughly matches gestational age in weeks after 20 weeks — so 28cm at 28 weeks is expected. In twin pregnancies, the uterus often measures ahead because two babies are taking up more space. A consistently larger-than-expected measurement is one of the things that prompts a provider to order an ultrasound.

The total weight a woman gains between before pregnancy and delivery.

Gestational weight gain is the difference between a woman's weight before pregnancy and her weight just before delivery. Recommended ranges depend on pre-pregnancy BMI and on whether she is carrying one baby or more than one. For twin pregnancies, providers individualize and monitor targets, because both gaining too little and gaining too much have been linked to complications.

An ultrasound to measure how well a baby is growing.

Growth scans use ultrasound to measure fetal size and estimate weight, typically by measuring head circumference, abdominal circumference, and femur length. In twin pregnancies, growth scans are performed more frequently than in singleton pregnancies — typically every 4 weeks for dichorionic twins and every 2 weeks for monochorionic twins — to detect growth discordance or restriction early.

The hormone produced in pregnancy — the one pregnancy tests detect.

Human chorionic gonadotropin (hCG) is a hormone produced by the placenta shortly after a fertilized egg implants in the uterine lining. It is the hormone detected by home pregnancy tests. In twin pregnancies, hCG levels are typically higher than in singleton pregnancies because two placentas — or a larger shared placenta — produce more of it. A very strong positive on an early pregnancy test, or unusually high hCG on a blood test, can be an early indicator of twins — though only an ultrasound can confirm.

Releasing more than one egg in a single cycle — the mechanism behind fraternal twins.

Hyperovulation is the release of two or more eggs during a single menstrual cycle. When both eggs are fertilized by separate sperm, fraternal (dizygotic) twins result. Hyperovulation is influenced by genetics — it runs on the maternal side of families — as well as by age (women over 30 are more likely to hyperovulate), body size, and fertility medications. It is the primary biological mechanism behind fraternal twinning and is not related to the cause of identical twins, which results from a single fertilized egg splitting.

A group of conditions involving high blood pressure during pregnancy.

Hypertensive disorders of pregnancy include chronic hypertension, gestational hypertension, and preeclampsia. Twin pregnancies carry a higher risk than singleton pregnancies, which is one reason blood pressure is checked at every prenatal visit.

A hormone linked to cell growth — and to higher fraternal twin rates in taller women.

Insulin-like growth factor (IGF-1) is a hormone that promotes cell growth and is influenced by nutrition, body size, and genetics. Research has suggested that taller women tend to have higher IGF-1 levels, and that higher IGF-1 may stimulate the ovaries to release more than one egg per cycle — contributing to a slightly elevated rate of fraternal twinning in taller women. The association is modest and population-level, not a reliable individual predictor.

When a baby grows more slowly than expected in the womb.

IUGR occurs when a fetus does not grow at the expected rate inside the uterus, typically defined as an estimated weight below the 10th percentile for gestational age. In twin pregnancies, growth restriction is more common than in singleton pregnancies, particularly in monochorionic twins where one twin may receive less placental blood flow. Regular growth scans are a key part of twin pregnancy monitoring.

A state in which the body burns fat for fuel and produces ketones.

Ketosis occurs when the body does not have enough carbohydrate available and breaks down fat for energy, producing ketones. In pregnancy, eating enough carbohydrate helps avoid it, which is one reason twin-pregnancy nutrition guidance sets a daily carbohydrate minimum.

A baby whose birth weight is above the 90th percentile for gestational age.

Large for gestational age usually means a birth weight above the 90th percentile for a baby's gestational age. In studies of weight gain in pregnancy, gaining more than the recommended range has been associated with a higher chance of an LGA baby.

An obstetrician with specialist training in high-risk pregnancies.

A maternal-fetal medicine specialist — also called a perinatologist — is an obstetrician who has completed subspecialty training in managing complex or high-risk pregnancies. Monochorionic twin pregnancies, and many dichorionic twin pregnancies, are typically co-managed or referred to an MFM specialist due to the increased surveillance and intervention risks involved. If your OB does not have significant twin experience, an MFM referral is appropriate and worth requesting.

Both twins share a single amniotic sac — the highest-risk twin arrangement.

Monoamniotic describes a twin pregnancy in which both babies share one amniotic sac, with no separating membrane between them. All monoamniotic twins are also monochorionic (sharing a placenta). The primary risk is umbilical cord entanglement, which can restrict blood flow to one or both twins. Monoamniotic (Mo-Mo) pregnancies represent roughly 1–2% of twin pregnancies and are typically managed with hospitalization from around 28 weeks and early caesarean delivery at 32–34 weeks.

Having one shared chorion — meaning twins share a single placenta.

Monochorionic describes a twin pregnancy in which both babies share a single chorion and a single placenta. All monochorionic twins are identical (monozygotic). Monochorionic twins account for roughly 20% of all twin pregnancies but carry a higher risk of complications — including TTTS, TAPS, and selective growth restriction — due to shared placental blood vessels. Monochorionic pregnancies require specialist surveillance, typically every two weeks from 16 weeks.

Identical twins who share a placenta but have separate amniotic sacs — the most common identical twin arrangement.

Mo-Di twins are monozygotic (identical) twins who share a single placenta but each have their own amniotic sac. This is the most common arrangement among identical twins, occurring when the embryo splits between days 4 and 8. Mo-Di pregnancies require specialist monitoring — typically every two weeks from around 16 weeks — due to the risk of twin-to-twin transfusion syndrome (TTTS) and other placental complications.

Identical twins who share both a placenta and an amniotic sac — the highest-risk twin arrangement.

Mo-Mo twins share both a placenta and a single amniotic sac. This occurs when the embryo splits after day 8, and represents the rarest and highest-risk twin arrangement. The primary risk is umbilical cord entanglement. Mo-Mo pregnancies typically require hospitalization from around 28 weeks and are delivered early, usually by caesarean section around 32–34 weeks.

A birth defect of the brain, spine, or spinal cord that forms in early pregnancy.

Neural tube defects, such as spina bifida, occur when the structure that becomes the brain and spinal cord does not close fully in the first weeks of pregnancy. Taking folic acid before conception and in early pregnancy lowers the risk.

A first-trimester ultrasound that measures fluid at the back of the baby's neck as a screening marker.

The nuchal translucency (NT) scan is typically performed between 11 and 14 weeks of pregnancy. It measures the fluid-filled space at the back of the fetal neck — increased NT is associated with chromosomal abnormalities including Down syndrome. In twin pregnancies, it is also one of the best opportunities to accurately assess chorionicity and amnionicity, making first-trimester ultrasound timing particularly important.

The number of times a person has given birth — a factor in twin likelihood.

In obstetrics, parity refers to the number of times a person has given birth to a baby at or beyond 20 weeks of gestation, regardless of outcome. Higher parity — having had more previous pregnancies — is associated with a modestly increased likelihood of fraternal twins, possibly because the body becomes more efficient at releasing multiple eggs with each successive pregnancy. A person who has had two or more previous pregnancies is considered higher parity.

A specialist obstetrician focused on high-risk pregnancies — also called a maternal-fetal medicine (MFM) specialist.

A perinatologist is an obstetrician who has completed subspecialty fellowship training in maternal-fetal medicine (MFM) — the branch of obstetrics focused on high-risk pregnancies. They manage complex conditions including monochorionic twin pregnancies, preeclampsia, preterm birth risk, and fetal abnormalities. For monochorionic twin pregnancies, care involving a perinatologist is standard — both for the biweekly surveillance protocol and for management if complications such as TTTS or sFGR develop.

The organ that supplies oxygen and nutrients from mother to baby during pregnancy.

The placenta is a temporary organ that develops during pregnancy, attaching to the wall of the uterus and connecting to the baby via the umbilical cord. It transfers oxygen and nutrients from the mother's blood to the baby, and removes waste products in the other direction. In twin pregnancies, whether twins share a placenta (monochorionic) or have separate placentas (dichorionic) is the primary determinant of pregnancy risk and monitoring requirements.

A pregnancy complication involving high blood pressure and potential organ damage.

Preeclampsia is a serious pregnancy complication characterized by high blood pressure and signs of organ damage, most commonly affecting the liver and kidneys. It typically develops after 20 weeks of pregnancy. Twin pregnancies carry a significantly higher risk of preeclampsia than singleton pregnancies — roughly double the risk. It requires close monitoring and may necessitate early delivery.

Birth before 37 weeks of pregnancy.

Preterm birth is defined as delivery before 37 completed weeks of gestation. It is significantly more common in twin pregnancies — more than 50% of twins are born preterm, compared to approximately 10% of singletons. The average gestational age at delivery for twins is around 35–36 weeks. Preterm birth is the primary reason twin pregnancies carry higher NICU admission rates and why monitoring and birth planning differ from singleton pregnancies.

When one monochorionic twin grows significantly more slowly due to unequal placental share.

Selective fetal growth restriction (sFGR) is a complication of monochorionic twin pregnancies in which one twin receives a disproportionately small share of the shared placenta, resulting in significantly slower growth than the co-twin. Defined by an estimated weight discordance typically exceeding 25%, with the smaller twin below the 10th percentile. sFGR is classified into three types based on umbilical artery Doppler flow patterns, which guide management. Severe cases may require fetal intervention.

A procedure to reduce the number of fetuses in a multiple pregnancy.

Selective reduction (also called multifetal pregnancy reduction) is a medical procedure performed in the first or early second trimester to reduce the number of fetuses in a high-order multiple pregnancy (triplets, quadruplets, or more) to reduce health risks. It can also be performed in twin pregnancies when one fetus has a serious abnormality. It is a topic that should be discussed before IVF transfer, not during the pregnancy.

Growth restriction affecting one twin but not the other in a monochorionic pregnancy.

sIUGR occurs specifically in monochorionic twin pregnancies when one twin's growth is significantly restricted while the other grows normally, due to unequal sharing of the placenta. It is defined by a discordance in estimated fetal weights — typically when one twin is below the 10th percentile and the difference between twins exceeds 25%. Management depends on severity and may range from increased monitoring to early delivery.

A baby whose birth weight is below the 10th percentile for gestational age.

Small for gestational age usually means a birth weight below the 10th percentile for a baby's gestational age. Many SGA babies are healthy and simply small, but SGA can also reflect restricted growth in the womb. Twins are more likely than singletons to be born small, and some studies use twin-specific growth references.

The leading US professional body for high-risk pregnancy specialists, which sets twin monitoring standards.

The Society for Maternal-Fetal Medicine (SMFM) is the primary professional organization for maternal-fetal medicine specialists (perinatologists) in the United States. SMFM publishes clinical guidelines and consult series that define the standard of care for high-risk pregnancies, including the biweekly surveillance protocol for monochorionic twin pregnancies. Website: https://www.smfm.org

An ultrasound marker indicating monochorionic (shared placenta) twins.

The T-sign describes the appearance of the inter-twin membrane on ultrasound in a monochorionic pregnancy — a flat, T-shaped insertion into the placenta with no placental tissue wedging between the membrane layers. The absence of the triangular wedge (twin peak sign) distinguishes monochorionic from dichorionic placentation. Like the twin peak sign, the T-sign is most accurately assessed between 10 and 14 weeks.

A placental complication in monochorionic twins involving unequal red blood cell levels.

TAPS is a form of chronic blood transfusion between monochorionic twins, resulting in severe anaemia in the donor twin and polycythaemia (excess red blood cells) in the recipient. Unlike TTTS, amniotic fluid levels remain normal, making TAPS harder to detect on standard ultrasound. It can occur spontaneously or after laser treatment for TTTS. Doppler blood flow monitoring is used to screen for it.

A serious complication of monochorionic twin pregnancies where blood flows unequally between twins.

TTTS occurs in monochorionic (shared placenta) twin pregnancies when abnormal blood vessel connections in the placenta cause one twin (the donor) to give too much blood to the other (the recipient). The donor becomes anaemic and growth-restricted; the recipient receives excess blood and can develop heart complications. TTTS occurs in approximately 10–15% of monochorionic pregnancies. Treatment options include laser ablation of the connecting vessels and, in some cases, amnioreduction. Early detection through regular ultrasound surveillance is critical.

An ultrasound marker indicating dichorionic (separate placenta) twins — also called the lambda sign.

The twin peak sign — also called the lambda sign — is a triangular wedge of placental tissue visible on ultrasound projecting between the layers of the inter-twin membrane at its placental insertion. Its presence indicates dichorionic placentation (separate placentas). It is most reliably identified between 10 and 14 weeks of gestation. After 14 weeks, accuracy of chorionicity assessment declines significantly, making first-trimester ultrasound timing critical.

The gap between twins' estimated weights on ultrasound, shown as a percentage of the larger twin's weight.

Twin weight discordance, or estimated fetal weight (EFW) discordance, is calculated as (larger twin's EFW − smaller twin's EFW) ÷ larger twin's EFW × 100. Some difference is normal. Care teams watch the trend across scans, and thresholds of 20% and 25% are commonly used to trigger closer monitoring or specialist review. Discordance is a signal to look closer, not a diagnosis.

An ultrasound check of blood flow in the umbilical cord, used to see how well the placenta is supplying a baby.

Umbilical artery Doppler uses ultrasound to measure blood flow through the umbilical cord. Abnormal results can suggest the placenta is not supplying a baby as well as it should. In twin pregnancies it is often added when there is a weight gap between the twins or when one baby is measuring small.

A quick assessment of a newborn's health at 1 and 5 minutes after birth.

The Apgar score assesses five signs in the first minutes after birth: skin colour, heart rate, reflex response, muscle tone, and breathing effort. Each is scored 0–2, giving a total of 0–10. A score of 7 or above is generally considered reassuring. Premature twins may have lower initial Apgar scores simply due to prematurity rather than underlying health problems.

A chronic lung condition seen in babies born very early who need breathing support for a long time.

Bronchopulmonary dysplasia (BPD) is a chronic lung condition that mainly affects babies born very prematurely. It is usually diagnosed when a baby still needs oxygen or breathing support at 36 weeks' corrected age. Many babies improve as their lungs grow, though some need ongoing follow-up.

Surgical delivery of a baby through an incision in the abdomen and uterus.

Twin pregnancies have significantly higher rates of caesarean delivery than singleton pregnancies — though vaginal birth is possible and achieved by many twin families, particularly when the presenting twin is head-down (vertex). Monochorionic monoamniotic twins are almost always delivered by caesarean. The decision depends on the position of both twins, gestational age, fetal wellbeing, and the experience of the delivery team.

The practice of placing twin newborns in the same cot or incubator.

Co-bedding involves placing twins together in a shared sleep space, typically in the NICU or at home in the early weeks. Some research suggests it may have benefits for temperature regulation and weight gain in premature twins, possibly reflecting the closeness they shared in utero. Current safe sleep guidelines (which recommend back sleeping in a clear, separate sleep space) generally advise against co-bedding once babies come home, though practice varies.

A premature baby's age calculated from their due date rather than their birth date.

Corrected age (also called adjusted age) accounts for prematurity when assessing a baby's development. A baby born at 32 weeks who is now 6 months old has a corrected age of approximately 3 months, since they were born 3 months early. Developmental milestones for premature twins should be assessed using corrected age until at least 2 years. Most premature babies catch up to their peers by age 2–3.

How far along a pregnancy is, measured from the first day of the last menstrual period.

Gestational age is measured in weeks and days from the first day of the last menstrual period (LMP), meaning pregnancy is counted as starting approximately two weeks before fertilization actually occurs. Full term is 39–40 weeks for singletons; for twins, 37–38 weeks is considered term, reflecting that most twin pregnancies deliver earlier. Knowing gestational age at delivery is important for interpreting a premature twin's development and corrected age.

Skin-to-skin contact between a parent and newborn, especially beneficial for premature babies.

Kangaroo care involves holding a newborn — particularly a premature or low birth weight baby — skin-to-skin against a parent's chest. Research consistently shows it stabilizes heart rate, temperature, and breathing, promotes breastfeeding, and supports bonding. For NICU twins, kangaroo care can be done with both twins simultaneously (tandem kangaroo care), which families and nursing staff report as particularly powerful.

A birth weight below 2,500 grams (5.5 pounds).

Low birth weight is defined as a birth weight below 2,500 grams. It is more common in twins than singletons — partly because twins are often born preterm, and partly because of the shared resource demands of a twin pregnancy. Very low birth weight (below 1,500g) and extremely low birth weight (below 1,000g) are associated with higher risks of medical complications and require NICU care.

A specialist hospital unit for newborns who need extra medical support.

The NICU provides intensive medical care for premature babies and newborns with serious health conditions. Twin babies are admitted to the NICU at significantly higher rates than singletons — primarily because of higher preterm birth rates. NICU admission does not necessarily indicate a serious long-term health problem; many premature twins spend time in the NICU for monitoring and support before going home healthy.

Keeping newborns in the same room as their parents during a hospital stay.

Rooming in refers to the practice of keeping newborn babies in the same hospital room as their parents rather than in a central nursery. For twin families whose babies do not require NICU care, rooming in supports early bonding, breastfeeding establishment, and learning each baby's cues. Some parents find rooming in with two newborns exhausting — asking for NICU or nursery support overnight is a reasonable request and not a reflection on parenting.

Placing a newborn directly on a parent's bare chest immediately after birth.

Skin-to-skin contact — also called kangaroo care when practiced over extended periods — involves placing a newborn on a parent's bare chest. It stabilizes the baby's heart rate, temperature, and blood sugar, promotes breastfeeding, and supports bonding. For twin births, skin-to-skin is increasingly practiced even in operative deliveries. NICU protocols in many hospitals now support skin-to-skin contact for premature twins as early as their condition allows.

The time between the birth of the first and second twin in a vaginal delivery.

In a vaginal twin birth, there is a period between the delivery of the first and second twin during which the second twin must be monitored carefully. Most guidelines suggest the second twin should be delivered within 30 minutes of the first, though this varies by clinical circumstance. The position and wellbeing of the second twin are continuously assessed during this interval.

Deliberately seeking differences from a sibling to build a distinct identity.

Deidentification is the process of choosing different interests, styles, friends, or goals from a sibling in order to establish a separate identity and reduce competition. It is often contrasted with identification, in which siblings seek similarities. Research on college-age twins suggests deidentification can be gradual and can coexist with wanting to keep closeness and support.

The period between adolescence and settled adult roles, roughly ages 18 to 25.

Emerging adulthood describes the years, roughly 18 to 25, when many people are exploring identity, education, work, and relationships before taking on settled adult roles. For twins, this can be a time of decisions about whether to live, study, or work apart from a co-twin. Findings about this age group do not necessarily apply to twins at other stages of adult life.

How identical twins can differ biologically despite sharing nearly identical DNA.

Epigenetics refers to changes in gene expression that do not involve changes to the underlying DNA sequence. Even identical (monozygotic) twins, who start with essentially the same DNA, accumulate epigenetic differences over their lifetimes due to different environments, experiences, and exposures. These differences help explain why identical twins can diverge in health, behaviour, and even appearance as they age — and why twin studies remain a valuable tool for separating genetic from environmental influences.

Pregnancies with three or more babies — triplets, quadruplets, and beyond.

Higher-order multiples refers to pregnancies involving three or more fetuses. Triplet and higher pregnancies carry significantly elevated health risks compared to twin pregnancies — for both the mother and the babies — and are a key reason why clinical guidelines strongly discourage transferring more than two embryos in IVF. The rate of higher-order multiples in IVF has declined substantially as single embryo transfer practice has increased.

The developmental process through which twins establish separate identities.

Individuation is the psychological process by which a person develops a sense of self as a separate individual. For twins, this process is complicated by the constant presence of a co-twin and the social tendency to treat twins as a unit. Parenting practices that support individuation — separate friendships, solo experiences, individual recognition — are consistently recommended by twin development researchers. Healthy individuation benefits both twins and the relationship between them.

A temperament trait describing how easily and intensely a child becomes upset.

Negative emotionality is one dimension of temperament. Children high in it tend to get upset, fussy, or distressed more easily and more strongly. In studies of twins, it is usually measured by parent report, and researchers have examined whether growing differences between co-twins on this trait go along with more conflict between them.

When twins play near each other but independently — an important developmental stage.

Parallel play — where young children play alongside each other without directly interacting — is a normal developmental stage typically seen between ages 2 and 3. In twins, parallel play is often observed earlier and for longer because of constant proximity to a same-age peer. It should not be mistaken for a lack of connection — it is a normal phase that gradually evolves into cooperative play.

A person born as an only child of a pregnancy — not a twin or multiple.

In the context of multiple pregnancy, a singleton refers to a baby born alone — not as part of a twin or higher-order multiple pregnancy. The term is also used to describe a later pregnancy after twins: 'they had twins and then a singleton.' Singleton pregnancies carry lower baseline risks than twin pregnancies across most clinical measures.

A child's basic, early-appearing style of emotion, activity, and reaction to the world.

Temperament describes inborn-leaning differences in how children experience and express emotion and energy, such as how easily they get upset, how sociable or shy they are, and how active they are. It appears early, tends to stay fairly stable over time, and is partly shaped by genes. In twin research, differences in temperament between co-twins are studied as one influence on how close or conflicted the twin relationship is.

The close relationship between twins, shaped by shared genetics and a uniquely shared early experience.

The twin bond refers to the relationship between twins — which is shaped from conception by shared environment (in identical twins, shared genetics too) and proximity. Research by twin psychologists including Dr. Joan Friedman and Dr. Barbara Klein emphasizes that the twin bond, while often close, is not homogeneous — twin relationships vary widely in intensity, dependency, and dynamics. Supporting each twin's individual identity while honoring the bond is a core challenge of twin parenting.

The tendency for twins to amplify each other's behaviour — positive or negative.

Twin escalation syndrome describes the phenomenon where twins feed off each other's energy and escalate behaviour — whether excitement, silliness, defiance, or distress — more rapidly and intensely than either would alone. It is a recognized pattern in twin parenting literature and can make discipline, transitions, and overstimulation more challenging. Managing the environment (separating twins when escalation begins, planning activities with adequate downtime) is more effective than focusing on consequences after escalation has occurred.

A private language or communication system developed between twins.

Cryptophasia refers to the phenomenon of twins developing a private speech system — including invented words, phrases, or non-verbal signals — that only they understand. It is more common in identical twins and in twins who spend most of their time together. While often charming, persistent cryptophasia that delays the development of standard language may warrant speech and language assessment. Most twin language disappears naturally as children develop conventional communication skills.

A twin who has lost their co-twin at any point in life.

A twinless twin is a person whose twin has died — whether through pregnancy loss, infant death, childhood illness, or death at any stage of life. The grief experience of a twinless twin is considered distinct from other bereavement, involving the loss of a person who may have been the closest relationship of one's life and a fundamental part of one's identity. Organizations including the Twinless Twins Support Group International provide peer support for this community.

The distinctive grief experience of a twin who has lost their co-twin.

The grief of a twinless twin is considered by bereavement specialists to be among the most complex forms of loss — involving not just the death of a close person but the loss of a shared identity, a lifelong companion, and in many cases the person who has known them most deeply. Twinless twin grief can resurface at milestone moments throughout life — birthdays, achievements, transitions — and may not follow conventional grief timelines. Peer support from other twinless twins is widely reported as uniquely helpful.

The scientific field studying how genes and environment together shape behavior, personality, and psychological traits.

Behavioral genetics uses twin studies, adoption studies, and molecular genetic methods to estimate how much of the variation in human traits — intelligence, personality, mental health, interests, and more — can be attributed to genetic versus environmental factors. Twin studies are its most powerful tool: by comparing identical and fraternal twins raised together or apart, researchers can disentangle genetic from shared and non-shared environmental influences. The Minnesota Study of Twins Reared Apart, conducted 1979–1999, is the field’s landmark investigation. A core finding of behavioral genetics is that most complex psychological traits show substantial heritability — and that the environment’s influence often works through genetic predispositions rather than independently of them.

A widely used parent questionnaire that measures children's emotional and behavioural difficulties.

The Child Behavior Checklist is a standardized questionnaire in which a parent rates how true a list of behaviours is of their child. It produces scores for inward-directed difficulties such as anxiety, sadness and withdrawal; outward-directed difficulties such as aggression and rule-breaking; and an overall total. Separate versions exist for preschool and school-age children. Because it relies on a parent's report, results reflect the rater's perspective as well as the child's behaviour, which is why researchers value input from more than one informant.

The probability that both twins will share a particular trait or condition.

Concordance rate measures how often both members of a twin pair share a particular trait, disease, or outcome. In twin research, comparing concordance rates between identical and fraternal twin pairs helps estimate genetic contribution. A 100% concordance rate in identical twins for a condition would suggest it is entirely determined by genetics; lower rates suggest environmental factors also play a role.

Twins who differ on a particular trait, condition, or outcome.

Discordant twin pairs — particularly identical twins discordant for a disease or condition — are especially valuable in research because they allow scientists to study environmental influences while controlling for genetics. Famous examples include twins discordant for schizophrenia, autism, and certain cancers. Discordance in identical twins demonstrates that genes are not destiny — environment and chance play a real role.

A primary mechanism of epigenetic change — chemical tags added to DNA that affect which genes are expressed.

DNA methylation involves the addition of a methyl group to a specific location on the DNA molecule, typically at cytosine bases in CpG sequences. These chemical tags act as switches, generally silencing the genes they mark. Methylation patterns are influenced by age, environment, diet, and other exposures, and can change over a lifetime. In twin research, DNA methylation is the primary metric used to measure epigenetic divergence — studies comparing methylation patterns in identical twins have demonstrated that biological profiles become increasingly distinct as twins age and accumulate different life experiences.

The gradual divergence of gene expression patterns in identical twins over time, driven by different environments and experiences.

Epigenetic drift describes the accumulation of differences in DNA methylation and other epigenetic markers between identical twins over the course of their lives. Because epigenetic patterns are shaped by environment, diet, stress, exercise, and other exposures, twins who live different lives progressively develop different gene expression profiles — even though their underlying DNA sequence remains essentially identical. A landmark 2005 study published in PNAS documented this process across twins ranging in age from 3 to 74, finding that older twins, particularly those who had lived apart, showed substantially greater epigenetic divergence than young twins who had shared their environments. Epigenetic drift is one of the primary mechanisms by which identical twins can develop different health outcomes.

The study of how genes are switched on or off — without changes to the DNA sequence itself.

Epigenetics refers to changes in gene expression — which genes are active and which are silenced — that do not involve alterations to the underlying DNA sequence. These changes are driven by molecular mechanisms including DNA methylation and histone modification, and can be influenced by environment, diet, stress, and other external factors. In twin research, epigenetics helps explain how identical twins with essentially the same DNA can diverge biologically over time. The word comes from the Greek 'epi,' meaning 'above' or 'on top of.'

A large-scale study scanning the genome for genetic variants linked to traits or diseases.

A GWAS scans the genomes of large numbers of people to identify genetic variants — typically single nucleotide polymorphisms (SNPs) — associated with particular traits or diseases. Twin cohorts are particularly valuable in GWAS research because zygosity is known and genetic similarity can be precisely quantified. Many large-scale health studies, including the UK Biobank, include substantial numbers of twins.

The proportion of variation in a trait that is attributable to genetic factors.

Heritability is a statistical estimate — derived in large part from twin studies — of how much of the variation in a trait within a population can be attributed to genetic differences. A heritability of 0.8 for a trait means roughly 80% of the variation in that trait across the population is explained by genetic differences. Heritability is a population-level statistic, not an individual-level one — it doesn't mean 80% of your version of the trait is genetic.

Genetic segments that two people share because they inherited them from a common ancestor.

Identical by descent refers to genetic sequences that two individuals share because they both inherited them from the same ancestor — rather than by chance similarity. In twin research, the proportion of the genome shared IBD is a key metric: monozygotic twins share essentially 100% IBD, while dizygotic twins share approximately 50% — the same as full siblings. IBD analysis is used in complex genetic studies to understand disease inheritance patterns.

A study that follows the same people over time.

A longitudinal study measures the same people repeatedly over months or years, which lets researchers see how things change and whether one thing tends to come before another. In twin research, longitudinal studies follow twin pairs from early childhood into later years to track how their relationships and development change. They show associations over time but still cannot prove cause and effect on their own.

Identical twins can have significantly different gut bacteria despite sharing DNA.

Research has shown that even identical twins can have substantially different gut microbiomes — the community of bacteria and other microorganisms living in the digestive system. This is significant because the microbiome influences immune function, metabolism, and even mental health. Microbiome divergence in twins is driven by diet, environment, medication use, and chance — and provides evidence that health differences between identical twins are not solely genetic.

A landmark long-term study of identical twins raised apart, begun in 1979.

The Minnesota Study of Twins Reared Apart, led by Thomas Bouchard at the University of Minnesota, followed identical and fraternal twins who were separated in infancy and raised in different families. Its findings — that twins raised apart showed striking similarities in personality, interests, and health — were influential in establishing the heritability of a wide range of traits. The study ran from 1979 to 1999 and contributed more than 100 published papers.

The longstanding question of how much genes versus environment shape who we are.

The nature vs. nurture debate concerns the relative contributions of genetic inheritance (nature) and environmental influences (nurture) in shaping human traits, behaviour, and health. Twin studies — particularly those following identical twins raised apart — have been central to this research for over a century. The current scientific consensus is that virtually all traits are influenced by both genetic and environmental factors, and that the relationship between them is dynamic rather than fixed.

A statistic comparing the odds of an outcome between two groups.

An odds ratio compares the odds of an outcome in one group with the odds in another. A value above 1 means higher odds in the first group. An odds ratio is not the same as a risk ratio: when an outcome is common, such as preterm birth in twin pregnancies, an odds ratio overstates the change in risk. Studies usually report it with a 95% confidence interval showing the range of plausible values.

A database of twin pairs used for long-term health and genetics research.

A twin registry is a systematically maintained database of twins who have volunteered to participate in research. Major registries include the Swedish Twin Registry (the world's largest, with over 200,000 twins), the Australian Twin Registry, and the Mid-Atlantic Twin Registry in the US. Registries enable longitudinal studies — following the same twins over decades — and provide the statistical power needed to study complex traits and rare conditions.

Research that uses twins to understand the relative influence of genes and environment.

Twin studies compare identical (MZ) twins — who share nearly 100% of their DNA — with fraternal (DZ) twins — who share approximately 50% — to estimate the heritability of traits and conditions. If identical twins are more similar than fraternal twins on a given trait, that difference is attributed to genetics. Twin studies have generated foundational insights across medicine, psychology, and social science. They have also been criticized for oversimplifying the gene-environment relationship.

Unrelated children of nearly the same age raised together from infancy: twins in every way except shared genes.

Virtual twins are two unrelated children, born no more than about nine months apart, who join the same family in infancy and grow up side by side. Most are two adopted children, or an adopted child and a biological child of the same age. Twin researcher Dr. Nancy Segal has studied them through the Fullerton Virtual Twin Study at California State University, Fullerton since 1991. Because virtual twins share a home but not genes, comparing them with identical twins, who share both, helps researchers separate the influence of upbringing from the influence of genes.

Fraternal twins of different sexes — always dizygotic.

Boy-girl twins are always fraternal (dizygotic), since identical twins must be the same genetic sex. They share approximately 50% of their DNA — the same as any siblings. Boy-girl twins are sometimes incorrectly described as identical by parents and others; if twins are different sexes, they are definitively fraternal. They represent roughly one-third of all twin births.

A pushchair designed to carry two children simultaneously.

Double strollers come in two main configurations: tandem (one seat in front of the other) and side-by-side. Each has tradeoffs — tandems are narrower and easier to maneuver through doors; side-by-sides give both twins equal sightlines and are often easier for young children to board independently. Weight, foldability, and compatibility with infant car seats are key considerations. Many twin families use two separate single strollers as their children grow.

The national organization supporting local twins clubs across the United States.

Multiples of America — formerly the National Organization of Mothers of Twins Clubs (NOMOTC) — is the umbrella organization for local multiples clubs across the US. Local clubs provide community, peer support, and often a resale market for twin-specific gear. Membership in a local club is one of the most consistently recommended resources by experienced twin parents.

The additional support needs of parents in the weeks and months after bringing twins home.

Twin parents experience higher rates of postpartum depression, anxiety, and exhaustion than singleton parents — a finding consistent across multiple studies. The demands of caring for two newborns simultaneously, compounded by higher rates of NICU stays and preterm birth recovery, create a specific postpartum context. Structured help — scheduled visitors, overnight support, meal trains, and professional postpartum doulas — is not a luxury for twin families; it is a health recommendation.

The decision about whether twins should be placed together or apart at school.

School placement is one of the most frequently discussed decisions in twin parenting. Research does not support a universal recommendation — outcomes depend on the individual twins, their relationship, and the school environment. Current guidance from most twin specialist organizations recommends that parents advocate for annual review of placement decisions based on each child's current needs, rather than schools applying a blanket policy in either direction.

Twins who are both the same sex — male-male or female-female.

Same-sex twins may be identical or fraternal. Because sex cannot be used to determine zygosity, same-sex fraternal twins are often mistaken for identical twins — and vice versa. DNA testing is the only definitive way to determine zygosity in same-sex twins. Understanding zygosity matters for medical purposes, including assessing shared genetic risk for certain conditions.

The practice of placing twins in different classes at school.

The question of whether twins should be in the same or separate classrooms is one of the most common questions twin parents face at school age. Research is mixed, and expert opinion has shifted: most current guidance recommends following the individual twins' needs and preferences rather than applying a blanket policy. Some twins thrive together; others benefit from the social and academic space that separate classrooms provide. Many school systems now allow parents to make this choice annually.

Establishing healthy sleep habits and routines with two babies of the same age.

Sleep training with twins involves the same general principles as singleton sleep training — consistent routines, age-appropriate sleep associations, gradual independence — with the added complexity of synchronizing two babies' schedules. Most twin sleep consultants recommend synchronizing wake times from the start to build a shared schedule. The question of co-rooming vs. separate rooms, and whether one twin's crying will wake the other, is a common parental concern (research suggests twins habituate to each other's sounds fairly quickly).

Breastfeeding two babies at the same time.

Tandem breastfeeding — feeding both twins simultaneously, one at each breast — is practiced by many twin parents to manage feeding time and maintain supply. It requires positioning support (a twin nursing pillow is widely recommended) and often takes several weeks to establish. Many parents exclusively breastfeed twins; others use a combination of breastfeeding and formula. A lactation consultant with twin experience is a valuable resource in the early weeks.

The pattern of comparing twins to each other in ways that damage their individual sense of self.

Toxic twin comparison — a term used by Dr. Barbara Klein in her Psychology Today writing — refers to the persistent habit of parents, teachers, relatives, and strangers of measuring twins against each other: who is smarter, thinner, more social, more successful. Klein argues this pattern is damaging throughout the lifespan, not just in childhood, contributing to inadequacy and a distorted sense of individual identity. It is distinct from neutral observation of differences — toxic comparison frames one twin's traits as relative to or competitive with the other's.

The cultural fascination with twins as a phenomenon rather than as two individuals.

The twin mystique refers to the widespread cultural preoccupation with twins — particularly identical twins — as objects of fascination, curiosity, and projection. It is reflected in the tendency to ask twins intrusive questions, treat them as a unit, and project fantasies onto the twin relationship. Dr. Joan Friedman uses this term to describe the social force that works against twins' efforts at individuation: the world wants twins to be a spectacle, which makes the work of becoming two separate people harder.

How parents talk to and about twins affects their sense of individual identity.

Twin development researchers — particularly Dr. Joan Friedman — emphasize the importance of how parents talk to and about twins. Using each child's name rather than 'the twins,' avoiding labels like 'the smart one' or 'the social one,' and speaking about each twin's distinct preferences and interests all support healthy individuation. Language shapes identity, and the language around twins from early childhood has lasting effects.

The tendency — by parents, teachers, and others — to compare twins to each other.

Twin-to-twin comparison is one of the most consistently flagged challenges in twin development research. Being constantly compared to a sibling who is the same age — often in the same classroom, on the same team, living in the same home — can affect self-esteem and identity development. Twin development specialists recommend conscious effort to celebrate each twin's individual strengths and avoid framing one twin's progress in terms of the other's.

A leading online community and resource hub for parents of twins and multiples.

Twiniversity is a widely used online community, podcast, and resource platform founded by twin parent Natalie Diaz. It covers pregnancy, newborn care, toddler years, and beyond — with content specific to the twin parent experience. Many twin parents cite it as their primary source of peer support, particularly in the early weeks at home with newborns.

The psychological framework explaining how early bonds between caregiver and child form and shape development.

Attachment theory — originally developed by John Bowlby and expanded by Mary Ainsworth — describes how early emotional bonds between a caregiver and child form through repeated interactions: feeding, soothing, eye contact, and responsive care. These bonds provide the child with a secure base from which to explore the world. For twin parents, attachment theory is relevant because bonds naturally form sequentially with each child rather than simultaneously — a normal and well-documented pattern.

An unhealthy pattern where twins rely on each other at the expense of developing independent selves.

Co-dependence in twins describes a dynamic where each twin's sense of self, emotional regulation, or decision-making becomes excessively dependent on the other — beyond the normal closeness of the twin bond. Dr. Joan Friedman distinguishes healthy twinship closeness from co-dependence: the key marker is whether each twin has developed the psychological tools to be their own person when apart. Co-dependence often develops when twins are never given experiences of being separate, relying on themselves, or being recognized as individuals.

When a parent feels a stronger or earlier bond with one twin than the other.

Differential bonding refers to the common experience of parents forming bonds with their twins at different rates or with different intensity — particularly in the early weeks and months. It is not the same as having a favourite child and does not predict long-term parenting differences. Research on parent-child attachment shows differential feelings toward children are normal across all family configurations. With twins, the dynamic is more visible because both children are the same age and available for comparison simultaneously. Most parents find the differential resolves as each child's individual personality emerges.

The developmental process by which each twin establishes their own separate identity.

Individuation refers to the psychological process through which twins develop distinct senses of self, separate from their twin and from the unit of 'the twins.' Twin development specialists — particularly Dr. Joan Friedman — emphasize that healthy individuation is supported by parents treating each twin as a distinct individual: using their names rather than 'the twins,' allowing separate friendships and interests, and resisting the pressure to keep everything equal. Individuation is not about separating twins — it is about ensuring each child has the psychological space to know who they are.

Postpartum depression and anxiety occur at significantly higher rates in parents of twins and multiples.

Postpartum depression (PPD) and postpartum anxiety (PPA) affect a significantly higher proportion of twin parents than singleton parents, driven by compounded physical demands, higher rates of preterm birth and NICU stays, sleep deprivation, and the emotional complexity of bonding with two newborns simultaneously. PPD in twin parents is frequently underdiagnosed because the exhaustion and overwhelm are so expected as to seem normal. Persistent emotional distance from one or both children, intrusive thoughts, or inability to function beyond basic caregiving are signs worth discussing with a healthcare provider.

Distress experienced by twins when apart from their co-twin

Twins are particularly prone to separation anxiety due to the intense closeness formed before and after birth. Dr. Barbara Klein describes separation from a co-twin as a profound struggle at multiple life stages, stemming from the deep primary attachment twins develop in the womb. Managing separation gradually — giving twins experiences apart that they can tolerate — supports healthy individuation without unnecessary distress.

The sense of self a twin develops in relation to — and distinct from — their co-twin.

Twin identity refers to the dual psychological task facing every twin: forming an individual sense of self while also being part of a twin pair. Psychologists including Dr. Joan Friedman and Dr. Barbara Klein emphasize that healthy twin identity development requires each twin to be seen and treated as a distinct individual — not as half of a unit — from early childhood. The twin identity question ('who am I apart from being a twin?') is one that can resurface at developmental transitions throughout life.

The state and experience of being a twin

Twinship refers to the lifelong condition of being a twin — encompassing the unique psychological, emotional, and social dimensions of growing up and living as part of a twin pair. Researchers including Dr. Barbara Klein describe twinship as a distinct developmental experience shaped by both the twin bond and the quality of parenting received. See also: twin bond, twin identity.

Psychotherapist, twin expert, and author specialising in the emotional lives of twins and twin families.

Dr. Joan A. Friedman is a psychotherapist and one of the most widely cited experts on the emotional psychology of twins. She is an identical twin herself and the mother of fraternal twin sons — a combination that informs her clinical perspective in ways that are difficult to replicate. Her books include Emotionally Healthy Twins (2008) and The Same but Different: How Twins Can Live, Love, and Learn to Be Individuals (2014). She practices in Los Angeles and consults with twin families internationally. Website: https://www.joanafriedmanphd.com/

Behavioural geneticist, twin researcher, and author — one of the world's leading authorities on twin science.

Dr. Nancy L. Segal is Professor of Psychology at California State University, Fullerton, and Director of the Twin Studies Center she founded in 1991. She is one of the most prolific and widely cited twin researchers in the world, having contributed to the landmark Minnesota Study of Twins Reared Apart and authored nine books on twins. Her books include Born Together — Reared Apart (2012), Twin Mythconceptions (2017), and Accidental Brothers (2018). She is a recognised authority on zygosity, behavioural genetics, and the nature-versus-nurture question as illuminated by twin research. Website: https://drnancysegaltwins.org/

A shortage of healthy red blood cells, common in twin pregnancies due to increased demands on blood supply.

Anaemia occurs when the blood does not have enough healthy red blood cells to carry adequate oxygen to the body’s tissues and to the developing babies. It is more common in twin pregnancies because the body must supply two babies simultaneously, increasing iron and folate demands. Symptoms include fatigue, pallor, shortness of breath, and dizziness. Management typically involves dietary changes — iron-rich foods such as red meat, lentils, and leafy greens — and sometimes supplementation. Haemoglobin levels are monitored routinely at antenatal appointments.

An extremely rare condition in which a malformed cellular mass — the remnant of a parasitic twin — develops inside the body of the dominant twin.

Fetus in fetu (Latin for “fetus within a fetus”) is a rare variant of parasitic twinning in which an undeveloped twin becomes enclosed within the body of the dominant twin during early development. The internal mass is sustained by drawing from the host twin’s blood supply. Unlike an external parasitic twin, fetus in fetu may go undetected for years or decades. Cases have been discovered in infants during routine imaging and in adults with no prior awareness of the condition. Surgical removal is typically performed when the mass poses a risk to the host. It is estimated to occur in fewer than 1 in 500,000 births.

High blood sugar that develops during pregnancy, more common in twin pregnancies.

Gestational diabetes mellitus (GDM) occurs when the body does not produce or use enough insulin during pregnancy, resulting in elevated blood glucose levels. It is more prevalent in twin pregnancies than in singleton pregnancies, partly due to the increased metabolic demands of carrying two babies. Symptoms — increased thirst, frequent urination, and fatigue — are not always obvious, making routine screening important. Most cases are managed through dietary adjustments and monitoring; some require medication or insulin. GDM typically resolves after birth, but carries an increased risk of type 2 diabetes later in life.

A severe pregnancy complication involving liver and blood clotting problems, related to preeclampsia.

HELLP syndrome is a serious obstetric complication characterised by Haemolysis (breakdown of red blood cells), Elevated Liver enzymes, and Low Platelet count. It is considered a severe form of preeclampsia, though it can occur without classic preeclampsia symptoms such as high blood pressure. Symptoms may include nausea, upper abdominal or chest pain, shoulder pain, headache, and visual changes. HELLP is rare but potentially life-threatening and requires urgent medical attention. It most commonly develops in the third trimester or shortly after birth. Delivery of the baby is typically the definitive treatment.

A liver condition in pregnancy causing intense itching, due to a build-up of bile salts in the blood.

Obstetric cholestasis (also called intrahepatic cholestasis of pregnancy, or ICP) is a liver disorder in which the normal flow of bile is impaired, causing bile salts to accumulate in the bloodstream. The hallmark symptom is severe itching — most often on the hands and feet — without a rash. Other symptoms may include fatigue, dark urine, pale stools, and loss of appetite. It requires monitoring and sometimes early delivery to protect the baby. Keeping cool and reducing fatty foods may ease symptoms. Obstetric cholestasis usually resolves after birth.

Group of muscles supporting the bladder, bowel, and uterus

The pelvic floor is a group of muscles, ligaments, and connective tissue that form a hammock-like base at the bottom of the pelvis. These muscles support the bladder, bowel, and uterus, and play a key role in bladder control, sexual function, and core stability. Twin pregnancies place significant additional pressure on the pelvic floor, making postpartum rehabilitation especially important.

Therapeutic exercises to restore pelvic floor function after pregnancy

Pelvic floor rehabilitation is a specialized form of physiotherapy focused on restoring the strength, coordination, and function of the pelvic floor muscles following pregnancy or childbirth. For parents of twins, pelvic floor rehabilitation is particularly recommended before returning to impact exercise, given the extended physical demands of a multiple pregnancy.

A condition in which the internal organs are positioned as a mirror image of their normal arrangement.

Situs inversus is a congenital condition in which the major internal organs are located on the opposite side of the body from their typical position — for example, the heart on the right rather than the left, and the liver on the left rather than the right. In twins, it is associated with mirror image twinning, where a late embryonic split (typically 7–12 days after fertilization) results in one twin developing reversed organ placement. Situs inversus alone does not typically cause health problems, but associated cardiac variants (situs inversus with dextrocardia) require monitoring.

A chromosomal condition in females caused by a missing or incomplete X chromosome, associated with short stature and absent ovarian development.

Turner syndrome occurs when a female is born with only one complete X chromosome instead of two (written as 45,X or X0). Features commonly associated with the condition include short stature, absent or underdeveloped ovaries, potential hearing and cardiac issues, and infertility. In the context of twins, Turner syndrome can occur in rare cases of boy/girl identical (monozygotic) twins: a male embryo (XY) loses its Y chromosome very early in development, causing that twin to develop as female with Turner syndrome while the co-twin remains male. Turner syndrome affects approximately 1 in 2,000 to 2,500 female births.

Excessive psychological fusion between twins

A pattern in which twins' identities, emotional lives, and daily functioning are so intertwined that each twin has difficulty operating independently. Enmeshment can be comforting in childhood but becomes problematic in adolescence and adulthood when individuation is required. It is a common underlying factor in persistent twin rivalry and adult twin relationship difficulties.

Destructive resentment directed at a co-twin

A clinical distinction made by twin psychotherapist Dr. Joan Friedman. Unlike ordinary jealousy — wanting what a co-twin has — envy involves hostility and resentment toward the co-twin for having it. Envy in twins often goes unacknowledged and surfaces as persistent hostile behaviour. It typically requires professional support to resolve.

The process of developing a separate identity from one's co-twin

The psychological process by which each twin develops a distinct sense of self, separate from their identity as part of a twin pair. Individuation is a core developmental task for twins at every life stage, and inadequate individuation is a primary driver of rivalry, enmeshment, and conflict in twin relationships.

Academic or social rivalry between twins

The tendency for twins to compare themselves to each other academically, socially, or developmentally — particularly when in the same classroom or shared environment. Can be motivating or harmful depending on the twins' individual temperaments and the degree to which adults reinforce comparison.

Variation in how strongly individuals respond to their environment

The theory that people vary in how much their environment shapes their outcomes — not because some are weaker, but because their nervous systems are tuned to different levels of sensitivity. Highly susceptible individuals are more affected by both negative and positive experiences.

Experiences unique to each twin, even within the same family

The environmental influences that differ between individuals raised together. For twins, this includes different friendships, teachers, illnesses, and random events. Non-shared environment is a primary driver of why identical twins diverge over time.

Africa south of the Sahara Desert

The region of the African continent south of the Sahara Desert, comprising approximately 46 countries. Used in demographic and public health research to distinguish this region — which has distinct epidemiological patterns, including the world's highest natural twinning rates — from North Africa.

Twin births per 1,000 total births

The number of twin deliveries per 1,000 total deliveries in a given population. Used to compare twin birth frequency across regions, countries, and time periods. The global average was approximately 12 per 1,000 in 2010–2015.

When a gene's effect depends on environmental conditions

A gene-environment interaction occurs when the influence of a genetic variant depends on the environment a person experiences. The same gene can produce very different outcomes under different conditions.

A gene influencing sensitivity to stress and depression

A gene that regulates how the brain processes serotonin, a chemical involved in mood. People with a shorter version of this gene tend to show greater sensitivity to their environment and a wider range of mental health outcomes. One of the most studied genes in twin and stress-response research.

Brain region controlling hormones and basic body functions

A small region at the base of the brain that acts as the body\'s master regulator, controlling hormone release, body temperature, hunger, sleep, and the stress response. In twins research, the hypothalamus is relevant to studies on the HPG (hypothalamic-pituitary-gonadal) axis and the hereditary factors behind hyperovulation — the release of multiple eggs in a single cycle — which increases the likelihood of fraternal twins.

Placental blood flow imbalance in monochorionic twins

A condition affecting monochorionic twins in which blood flow through shared placental vessels becomes unbalanced, with one twin (the donor) transferring blood to the other (the recipient). TTTS occurs in approximately 10–15% of monochorionic pregnancies and is monitored through regular ultrasound scans. Treatment options for more severe cases include laser ablation of the connecting vessels.

Medical care during pregnancy

Healthcare provided to a pregnant person before delivery, including check-ups, screenings, and monitoring of fetal development. The WHO recommends a minimum of four antenatal visits; eight or more are recommended for higher-risk pregnancies including twins. Also called prenatal care.

Death rate for children under five years old

The probability that a child born in a given year will die before reaching age five, expressed per 1,000 live births. A standard measure of child health outcomes used by the WHO and UNICEF to compare populations globally.

A priest of Ifa divination in Yoruba religion, consulted on matters including twin births.

A babalawo is a priest of Ifa, the Yoruba divination system, responsible for interpreting the Ifa oracle and performing rituals tied to major life events. In twin tradition, a babalawo is traditionally consulted on the third day after a twin birth to protect the newborns, and again if a twin dies, to commission an Ere Ibeji figure.

A carved wooden figure made to house the soul of a deceased twin in Yoruba tradition.

Ere Ibeji (from "ibi," born, and "eji," two; "ere" means sacred image) is a small carved wooden figure commissioned by Yoruba families when a twin dies. Made by a local artisan at the direction of a babalawo, the figure is believed to house the departed twin's soul and is cared for as a living child — washed, fed, dressed, and never referred to as dead, but as having "traveled."

The Yoruba word for twins, and the divine twin spirits (Orisha) in Yoruba religion.

Ibeji is the Yoruba word for twins and the name given to the divine twin spirits in Yoruba religion. When a twin dies, families traditionally commission a small carved wooden figure — also called an Ibeji — believed to house the deceased twin's spirit. The figure is cared for as a living child: washed, dressed, fed, and included in family life, so the surviving twin does not suffer the spiritual imbalance of losing their other half. The Yoruba people have one of the highest twinning rates of any population, and Ibeji reflects a broader cultural view of twins as sacred rather than exceptional.

The Yoruba name for the secondborn twin — considered the elder of the two.

Kehinde (also spelled Kenny) is the Yoruba name given to the secondborn of a twin pair. The name derives from a phrase meaning "arriving after the other." Despite being born second, Kehinde is traditionally considered the elder twin, believed to have sent Taiwo out first to scout the world before following.

A deity within the Yoruba religious pantheon, subordinate to the supreme being Olodumare.

Orisha (also spelled Orisa or Orixa) are the secondary deities of traditional Yoruba religion, subordinate to the supreme creator Olodumare. Individual Orisha govern specific domains — Shango (thunder), Ogun (iron and war), and Orisha Ibeji (twins) among them. Orisha veneration spread through the African diaspora into traditions including Santería, Candomblé, and Umbanda.

The Yoruba name for the firstborn twin — considered the younger of the two.

Taiwo (also spelled Taiye or Taye) is the Yoruba name given to the firstborn of a twin pair. The name derives from a phrase meaning "having the first taste of the world." Despite being born first, Taiwo is traditionally considered the younger twin, believed to have been sent ahead by Kehinde to test whether the world was fit to enter.