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Frequently Asked Questions About Twins

“What are fraternal twins?”
Fraternal twins are the result of two separate eggs (ovum, or zugotes) becoming fertilized by two separate sperm, resulting in two completely distinct pregnancies in the womb at the same time. They are known as non-identical or dizygotic twins. Dizygotic twins account for about 2/3rds of all twins.

“What are identical twins?”
Identical, or monozygotic, twins occur when a single fertilized egg splits into two embryosaround the time the fertilized egg is becoming implanted in the womb (usually between the fourth and 12th day after fertilization). Monozygotic twins make up 1/3rd of all twins.

“How do you tell look-alike fraternals from true identical twins?”
A DNA test is required to determine with a high degree of certainty whether twins are identical or not. Blood-typing is a less certain realiable but sometimes used. Sometimes physicians will predict whether twins are identical based upon an examination of the placenta(s), but this is much less accurate.

“What are mirror-image twins?”
Identical twins, always monozygotic, can be different in one fascinating way—they may exhibit mirror-image features or behaviors. For example, they may have opposite hair whorls or opposite dominant hands or even mirror-image fingerprints. Mirror-imaging is related to the timing of the splitting of the fertilized egg. As many as 25% of all identical twins exhibit some kind of mirror-imaging.

“Are all same-sex twins (boy/boy or girl/girl) identical (monozygotic)?”

No, definitely not. This is tricky—onozygotic/identical twins are nearly always the same sex either boy/boy or girl/girl. Until very recently, in fact, experts thought monozygotic/identical twins had to be same-sex. But now we know that there are certain rare instances* when monozygotic/identical twins can actually be different-sex individuals; when this has occurred and been documented, the twins are genetically identical in every way other than a slight chromosomal difference.
Roughly one-third of all twins are boy/boy sets; another third are girl/girl sets; and the remaining one-third, roughly speaking, are mixed-sex boy/girl twins.

But make no mistake, not every set of boy/boy or girl/girl twins is monozygotic/identical. In fact, many same-sex sets of twins are dizygotic/fraternal twins. Roughly two-thirds of all twins in a same-sex set are dizygotic/fraternal. They sometimes look very much alike, but their physical appearances are not what makes a set of twins monozygotic/identical at all!

Very often, same-sex twins look very different, indeed, sometimes right from birth. They all have different-color hair and eyes, be of different height and have different physical builds, and display very different mannerisms.

This leaves about one-third of all same-sex sets of twins who are monozygotic/identical. The same percentages are thought to hold true for monozygotic/identical sets of girls and boys.

When it comes to triplets, quads, quints, and even larger higher-order multiples, the vast majority of these sets include at least one pair of monozygotic/identical twins, with one or more dizygotics/fraternals included in the set.

*The rare instance in which a set of boy/girl monozygotic/identical twins occurs is the result of Turner’s Syndrome, in which both individuals are actually XY boys, but one child loses the Y chromosome, yielding a baby who is XO. One twin appears externally to be a girl, but grows up infertile, short, and usually with a couple of other recognizable physical characteristics. Miscarriages occur frequently in such pregnancies, which is why these sets of twins are rare.

“What are the basic types of conjoined twins?”

  • Thoracopagus: Ventral or frontal union at the chest, often with a shared heart; most common form of conjoined twins; about 35% of all conjoined twins.
  • Omphalopagus: Ventral or frontal union at the abdomen, often with shared liver tissue; the highest survival rate; about 30% of all conjoined twins.
  • Pyopagus: Dorsal or rear union at pelvis; about 19% of all conjoined twins; never involves the heart or umbilicus.
  • Ischiopagus: Ventral or frontal union at the pelvis, often with shared intestines, bladders, genitals and kidneys; about 6% of all conjoined twins.
  • Parapagus: Lateral or side union with a variety of third and fourth limbs; conjoinment extends a variable distance upward; about 5% of all conjoined twins.
  • Craniopagus: Dorsal or rear union at the head; only about 2% of all conjoined twins; never involves the heart or umbilicus. Rachipagus: Dorsal or rear union at the spine.
  • Cephalopagus: Ventral or frontal union including the head and chest; two faces on the opposite sides of the head; do not survive; extremely rare.
  • Dicephalus: This refers to one body and two heads.


“Is the twin bond real?”

Yes! As any parent of twins will you tell and researchers have documented, twins bond with each other in special ways. This is especially true of identical twins who, after all, share the same genes.

The special bond between twins often shows up in infancy. Baby twins may exhibit similar eating, sleeping, and behavior habits. They may tend to sleep at the same time and, unfortunately, awake and cry at the same time. Some parents note that their infant twins seem to entertain each other while in their cribs. Conversely, when separated some twins become easily upset.

As twins become older, the special bond between them remains even as differences begin to emerge. Playing together is a key part of this bonding. Sometimes, twins develop their own unique language, with words and phrases that are only understandable to them. This is known as cryptophasia. Even in normal conversations, one twin may finish the sentence started by the other.

Because of the special relationship between twins, parents struggle with whether to separate twins into different classrooms when they begin school. While many schools recommend separation so that each child can develop a stronger sense of autonomy, most experts (and parents) believe each situation should be decided individually.

Even as twins mature and develop more independent lives, the twin bond can remain very strong. Adult twins often maintain regular, even daily, contact with each other. Interestingly, studies have revealed that twins reared apart, especially identical twins, exhibit very similar behavioral characteristics even after they become adults. Similarities between twins reared apart show up in their voices, gestures, fears and phobias, and a host of other characteristics.

“When does labor happen?”

In the majority of twin pregnancies, labor begins between 28 and 33 weeks. Techniques available to modern physicians have reduced many of the risks associated with multiple deliveries. The interim between delivery of the first baby and the second baby averages 17 minutes, but the duration can be shorter or as long as several hours or even days. The presence of two babies in the womb can lead to one or both being in a transverse or breech position. Deliveries by cesarean section (C-section) are more common with multiple pregnancies. Sometimes the first baby is delivered vaginally, and the second baby is delivered by C-section.

“Can I breastfeed my twins?”

Because breast milk provides the most complete nourishment for newborns, many mothers of twins choose to breastfeed. Although many moms worry there won’t be enough milk for two, this is generally not a problem. Frequent nursing stimulates milk production. During the first month, infants nurse 7 to 10 times a day. Smaller babies, which twins often are, usually need to nurse more frequently. Twins are often nursed simultaneously, but there’s certainly no guarantee the twins will want to nurse at the same time. Mothers elect or switch to bottle feeding for a number of reasons, especially if the breastfeeding schedule becomes exhausting. Even bottle feeding can be a challenge when both babies want to be fed at the same time. A number of inventive techniques (and special gadgets) have been developed to hold all the bottles and babies involved!

“What are some cool facts about twins?”

  • 18% to 22% of twins are left-handed compared with under 10% for non-twins.
  • The incidence of fraternal twins varies by race:
    • Of African descent: 1 birth per 70
    • Of Caucasian descent: 1 birth per 88
    • Of Japanese descent: 1 birth per 150
    • Of Chinese descent: 1 birth per 300
  • Genetic factors do not appear to have much affect upon the incidence of identical twins.
  • Identical twins exhibit almost identical brain wave patterns.
  • William Shakespeare was the father of boy/girl twins Hamnet and Judith. He also wrote about twins in The Comedy of Errors andTwelfth Night.
  • The term “twins” derives from the ancient German word twin or twine meaning “two together.”
  • Twinning does run in families, but supposedly only fraternal twins and not identical twins. Yet there are many, many families with identical twins in each generation, and sometimes even several sets of identical twin children, so this ostensible scientific “fact” doesn’t seem to be supported by the evidence. There is little evidence to support the popular notion that twins “skip” a generation.
  • If the mother is herself a fraternal twin, the chances of having twins increases about five-fold.
  • The modern world record for giving birth to multiples is held by Leontina Albina from Chile, whose 55 children included three sets of triplets. The all-time historical record is claimed by a Russian who is purported to have given birth to 6 pairs of twins, 6 sets of triplets, and 4 sets of quads. (If true, it means her 46 children included no singletons at all!)
  • The scientific study of twins is known as “gemellology.”
  • Twins have been known to develop their own “language” that only they understand. This process is known as “cryptophasia.”
  • The average time between the delivery of the 1st and 2nd twin is 17 minutes.
  • Multiples usually arrive a bit early and are born at 28 to 35 weeks of gestation (the average non-twin is born at 37 weeks).
  • If identical twin sisters or identical twin brothers bear children, the children are genetically half-siblings. It’s as if the woman had children with 2 separate men or if the man had chilren with 2 different women. If idential twin women marry identical twin men and both couples have children, the children are genetic siblings, but legally they are first cousins.
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