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.
- 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.
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.
- 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 and Twelfth 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.