Twins

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Everything about twins here

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Twins are two offspring resulting from the same pregnancy, either of the same or opposite sex.

The general term for more than one offspring from the same pregnancy (multiple birth) is multiples, for example triplets refers to cases of three offspring from the same pregnancy. A fetus alone in the womb is called a singleton.

Twins are usually, but not necessarily, born in close succession. Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks on average, 3 weeks less than full term.[1] Since premature births can have health consequences for the babies, twin births are often handled with special precautions.

Twins can either be monozygotic (MZ, colloquially, "identical") or dizygotic (DZ, colloquially, "fraternal"). There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the world population), and just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins).[2] The current rate in the United States is 31 twin births per 1,000 women.[3]

The Yoruba, a large west African ethnic group, have the highest rate of twinning in the world.

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Types of twins


There are five common variations of twinning. The three most common variations are all dizygotic: (1) male-female twins are the most common result, at about 40 percent of all twins born; (2) female DZ twins (sometimes called sororal twins); (3) male DZ twins. The other variations are monozygotic twins: (4) female MZ twins and (5) (least common) male MZ twins. Male singletons are slightly (about five percent) more common than female singletons. (6) There is also the mirror image variations. This is where the twins develop reverse asymmetric features. About 25% of monozygotic twins are mirror image twins.The rates for singletons vary slightly by country [4]. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Another variety of twins, "polar body twins," (one egg fertilized by two different sperm) is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. Polar body twinning would result in "half-identical" twins.[5

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Dizygotic twins
Dizygotic twins (commonly known as fraternal twins, but also referred to as non-identical twins or biovular twins) usually occur when two fertilized eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different sperm cells, DZ twins result. The two eggs, or 'ova', form two zygotes, hence the terms 'dizygotic' and 'biovular'.

Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, DZ twins may look similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to have the same age.

Studies show that there is a genetic basis for DZ twinning. However, it is only the female partner that has any influence on the chances of having DZ twins as the male cannot make her release more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.[6]

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[7] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

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Monozygotic twins

Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, the majority of monozygotic twins share the same placenta and amniotic sac,(although not always) while dizygotic twins do not.Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. Their traits and physical appearances are not exactly the same; although they have nearly identical DNA[1], environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[8] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization), each cell may develop separately its own placenta (chorion) and its own sac (amnion). These are called dichorionic diamniotic (di/di) twins, which occurs 20–30% of the time. Most of the time in MZ twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called monochorionic diamniotic (mono/di) twins.

In about one percent of MZ twinning the splitting occurs late enough to result in both a shared placenta and a shared sac called monochorionic monoamniotic (mono/mono) twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 50 percent, principally due to cord entanglement prior to 32 weeks gestation. If expecting parents choose hospitalization, mortality can decrease through consistent monitoring of the babies. Hospitalization can occur beginning at 24 weeks, but doctors prefer a later date to prevent any complications due to premature births. The choice is up to the parents when to start hospitalization. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of DZ and MZ twins.

Mono/di twins have about a 25 percent mortality due to twin-to-twin transfusion syndrome. Di/di twins have the lowest mortality risk at about nine percent, although that is still significantly higher than that of singletons.[9]

Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are always the same sex. On rare occasions, monozygotic twins may express different phenotypes, normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly [10] [11]). Monozygotic twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. If each member of one set of MZ twins reproduces with one member of another set of MZ twins then the resulting children would be genetic full siblings. It is estimated that there are around 10 million monozygotic twins and triplets in the world.



Identical twins.

[Batoul29]

My sisters are twins
but it's easy to make a difference between them