There are two main types of twins — identical (monozygotic) and fraternal (dizygotic) — but biology doesn’t stop there. Scientists have identified at least nine distinct types of twins, including rare variants like sesquizygotic (semi-identical), mirror image, and parasitic twins. Most people only ever hear about two. Here’s the full picture.

The vast majority of twin births fall cleanly into one of the two main categories. But a handful of rare twinning types exist at the edges of that framework — some confirmed in only a handful of documented cases, some still theoretical despite compelling evidence. If you’re a twin, a parent of twins, or just someone who’s always wondered why some things don’t quite fit the textbook explanation, this is the guide you didn’t know you needed.

The Two Main Types

Identical Twins (Monozygotic)

Identical twins form when a single fertilized egg splits into two embryos — and they share essentially 100% of their DNA.

Monozygotic twins develop when one egg is fertilized by a single sperm and the resulting embryo splits during the first two weeks after conception. Because both embryos come from the same genetic source, they carry the same chromosomes and are almost always the same sex. The rate of identical twinning is consistent worldwide at roughly 3 to 4 per 1,000 births — unlike fraternal twinning, it is not influenced by fertility treatments or family history.

That said, identical twins can diverge significantly over time. Environmental differences in the womb — blood flow, positioning, nutrition — produce measurable physical distinctions even before birth. Epigenetic changes accumulate across a lifetime. Identical twins can and do become quite different as they age, in everything from health to personality.

About one-third of monozygotic twins have separate placentas. Two-thirds share a single placenta — a configuration called monochorionic — while each maintaining their own amniotic sac, umbilical cord, and share of the placental mass. Monochorionic pregnancies carry higher risks, including Twin-Twin Transfusion Syndrome (TTTS). Source: ICOMBO.

Fraternal Twins (Dizygotic)

Fraternal twins form when two eggs are fertilized by two different sperm — they are genetically ordinary siblings who happen to share a womb.

Dizygotic twins develop when two eggs are released during a single ovulation and each is fertilized independently. Each twin has their own placenta. They share around 50% of their genes — the same genetic overlap as any two non-twin siblings. They can be any combination of sexes; roughly half of fraternal twin pairs are opposite-sex.

Fraternal twinning is influenced by maternal age, family history on the mother’s side, ethnicity, and the use of fertility treatments. The significant rise in twin births since the 1980s is largely attributable to assisted reproductive technology.

The majority of twin births worldwide are dizygotic. It is estimated that one-third of all twins are identical and two-thirds are fraternal. Source: ICOMBO.

The Rare Types

The types below are confirmed or theorized variants that fall outside — or at the edges of — the standard identical/fraternal framework. Some are subtypes within identical twinning. Some challenge the categories altogether.

Mirror Image Twins

Mirror image twins are identical twins whose physical features appear reversed — as if each is a reflection of the other.

This occurs when a fertilized egg splits later than usual, typically 7 to 12 days after conception — long enough for the embryo to have established a distinct left and right side. The result: twins with birthmarks on opposite sides of the body, hair whorls spiraling in opposite directions, one right-handed where the other is left-handed.

In rare cases, a mirror twin may develop situs inversus — a condition in which internal organs are positioned on the opposite side of the body from normal (for example, the heart on the right rather than the left). There is no genetic test for mirror imaging; it is identified through physical observation. Around a quarter of identical twins are thought to be mirror pairs, making this the most common entry on this list.

Sesquizygotic (Semi-Identical) Twins

Sesquizygotic twins are genetically identical on their mother’s side but share only about half their father’s genes — placing them between identical and fraternal in the genetic spectrum.

In 2019, the New England Journal of Medicine published a case that required scientists to coin an entirely new term. A boy-girl twin pair underwent genetic testing and produced results that fit neither standard category. The proposed mechanism: a single egg was fertilized simultaneously by two sperm — an event that would normally produce a non-viable triploid embryo. In this case, the extra genetic material appears to have been redistributed during early cell division, allowing two viable embryos to form, each carrying different paternal DNA.

The term sesquizygotic comes from the Latin for “one and a half.” The 2019 confirmation was only the second documented case in medical history — the first was reported in 2004. Researchers believe this type is vanishingly rare but likely underdiagnosed, since standard zygosity testing is not designed to detect it.

Sesquizygotic twins share 100% of their maternal DNA and approximately 50% of their paternal DNA. They are genetically more similar than fraternal twins but are not genetically identical.

Half-Identical (Polar Body) Twins

Polar body twins are theorized to share all of their mother’s genes but only half of their father’s — similar to sesquizygotic twins, but arising through a different mechanism.

When an egg matures, it produces a small byproduct called a polar body — a cell that normally degenerates and dies because it contains very little cytoplasm. ICOMBO and other research bodies have proposed that in some cases, particularly with older eggs, the polar body splits off abnormally: it grows larger, receives more nourishment, and survives long enough to be fertilized by a separate sperm.

Because the polar body and the original egg share the same maternal genes, twins produced this way would carry identical maternal DNA but different paternal DNA — genetically midway between identical and fraternal. DNA profiles from suspected polar body twin pairs are consistent with this pattern, but the mechanism has never been directly observed. It remains a well-supported hypothesis with no definitive test to confirm it.

Chimeric Twins

Chimerism in twins occurs when two separately fertilized eggs fuse early in development, producing one individual who carries two distinct sets of chromosomes.

Rather than two separate twins developing, the two embryos merge into a single person with genetically mixed cells. Some chimeric individuals are identified through unusual findings in blood work — more than one distinct blood type, or cells that are simultaneously XX and XY. Chimerism can also arise when fraternal twin pregnancies involve fused placentas, allowing the circulatory systems to intermix.

Fewer than 25 cases of true human chimerism have been confirmed, according to ICOMBO. It is considerably more common in other mammals, including cattle. Many human cases are likely never detected: a chimeric individual may show no outward signs and would only be identified through genetic testing prompted by an unrelated medical investigation.

Boy/Girl Identical Twins

In extremely rare cases, identical twins can be assigned different sexes at birth — the result of a chromosomal mutation, not a second egg.

Identical twins originate from a single fertilized egg carrying one pair of sex chromosomes — either XX (female) or XY (male). In a small number of documented cases, a mutation occurred very early in development in which one twin, originally male (XY), lost a Y chromosome. That twin then developed as female.

The female twin in such cases is typically diagnosed with Turner syndrome, a chromosomal condition associated with short stature, absent ovarian development, and potential hearing and cardiac issues. The co-twin remains chromosomally and anatomically male. These cases are exceptionally rare and represent a chromosomal event during embryonic development — distinct from gender transition, which is a social and medical process.

Superfetation Twins: Conceived at Different Times

Superfetation twins are conceived during separate menstrual cycles — sometimes up to 24 days apart — meaning they have different gestational ages even when born on the same day.

Under normal circumstances, a pregnancy triggers hormonal changes that halt ovulation. In superfetation, ovulation occurs again despite an existing pregnancy, and the newly released egg is fertilized. The result is a twin pregnancy where the fetuses are at different stages of development, detectable on ultrasound.

Superfetation is relatively common in certain animal species. In humans it is considered quite unusual, with only a small number of well-documented cases in medical literature. In rare cases, the births themselves are separated by days or weeks — a situation sometimes called interval birth when deliberate medical intervention is involved.

Because superfetation twins are conceived at different times, they carry different gestational ages even when delivered on the same day.

Heteropaternal Superfecundation: Different Fathers

Heteropaternal superfecundation occurs when fraternal twins are conceived from two different fathers — possible when a woman ovulates more than one egg and has more than one partner during her fertile period.

One egg is fertilized by sperm from one partner, the other by sperm from a second partner. This is only possible with fraternal twins; identical twins originate from a single egg and cannot have different fathers. DNA testing has confirmed documented cases, including a widely reported 1995 incident involving a laboratory error during IVF treatment at a Dutch clinic, where two men’s sperm were inadvertently combined.

Conjoined Twins

Conjoined twins are identical twins whose embryonic division was incomplete — they are born physically connected, sharing tissue, bone, or internal organs.

The location and extent of the connection varies widely: some twins share only a small bridge of skin, others share major organs. Conjoined twinning occurs in approximately 1 in 200 identical twin pregnancies, or about 1 in 85,000 births overall. Surgical separation is possible in many cases and depends on which structures are shared. Some conjoined twins choose not to pursue separation and live full lives joined together.

Parasitic Twins

A parasitic twin is a form of conjoined twinning where one twin fails to develop fully and becomes entirely dependent on the other’s body to survive.

The dominant twin — called the autosite — develops normally. The parasitic twin is severely underdeveloped and incapable of independent survival, dependent on the autosite’s cardiovascular system. It may appear as extra limbs, a partially formed structure, or abnormal tissue attached to the dominant twin’s body. In some cases the attachment is internal rather than external.

A related condition is fetus in fetu — Latin for “fetus within a fetus.” This occurs when a malformed mass of cells develops inside the body of the dominant twin, sustained by drawing directly from the host’s blood supply. Cases have been discovered in infants and in adults who had no prior awareness of the condition.

Surgical removal is typically recommended when the parasitic twin poses a developmental or health risk to the autosite. Outcomes for the dominant twin following surgery are generally positive.

Why the Full Picture Matters

The standard identical/fraternal framework captures the vast majority of twin births. But the rare types matter — not just for scientific completeness, but for the twins and families navigating them. A sesquizygotic boy-girl pair, for instance, would have been classified as fraternal under any testing protocol available before the 2010s. Knowing their true genetic relationship changes how researchers study them, and in some cases how twins understand themselves.

Chimerism, polar body twinning, and sesquizygotic twins collectively suggest that the line between “identical” and “fraternal” is less a clean boundary and more a spectrum — with unusual points scattered along it. Twin biology is still being mapped. The more closely scientists look, the more variation they find.

If you’re curious about how your own twinship fits into this picture, zygosity testing is accessible and affordable. A simple cheek-swab DNA test can determine whether same-sex twins are identical or fraternal — and may, in unusual cases, reveal something more surprising than either.

Frequently Asked Questions

How many types of twins are there?

There are two main types — identical (monozygotic) and fraternal (dizygotic) — but scientists have identified at least nine distinct twinning types. These include mirror image twins, sesquizygotic (semi-identical) twins, polar body twins, chimeric twins, superfetation twins, heteropaternal twins, conjoined twins, and parasitic twins. Most are extremely rare.

What is the rarest type of twin?

Parasitic twins are among the rarest, estimated to occur in approximately 1 in 1,000,000 live births. Sesquizygotic (semi-identical) twins are also exceptionally rare — only two confirmed cases appear in medical literature. True human chimerism has been identified in fewer than 25 cases worldwide.

Can identical twins be different sexes?

In extremely rare cases, yes. If a chromosomal mutation occurs very early in development and one twin loses a Y chromosome, that twin develops as female while the co-twin remains male. The female twin is typically diagnosed with Turner syndrome. These cases are exceptionally uncommon and represent a chromosomal event, not gender transition.

Can twins really have different fathers?

Yes — but only fraternal twins. Heteropaternal superfecundation occurs when two separately released eggs are fertilized by sperm from two different partners. Identical twins, originating from a single egg, cannot have different fathers. DNA testing has confirmed documented cases, including some arising from IVF laboratory errors.

What is the difference between conjoined twins and parasitic twins?

Both arise from incomplete separation of an identical twin embryo, but the degree of development differs significantly. Conjoined twins are two fully or substantially developed individuals who are physically connected. In parasitic twinning, one twin fails to develop fully and is entirely dependent on the other’s body — it cannot survive independently. Parasitic twinning is far rarer than conjoined twinning.

Sources

  • International Council of Multiple Birth Organisations (ICOMBO). “Zygosity for Twins.” icombo.org
  • Healthline Medical Team. “How Many Types of Twins Are There?” Reviewed by Mia Armstrong, MD. healthline.com, updated April 2023.
  • Gabbett MT et al. (2019). “Sesquizygotic Twinning.” New England Journal of Medicine. PubMed: 30840909
  • BabyMed. “Parasitic Twin and Vanishing Twin Syndrome.” babymed.com
  • National Library of Medicine / MedlinePlus Genetics. “Is the probability of having twins determined by genetics?” medlineplus.gov