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Twins are a form of Multiple Birth in which the Mother gives birth to two Offspring from the same Pregnancy , either of the same or opposite Sex . The general term for more than one offspring from the same pregnancy 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''. Human twins are two individuals who have shared the Uterus during a single pregnancy and 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. Since Premature Births can have health consequences for the babies, twin births are often handled with special precautions. 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 identical twins (roughly 0.2% of the world population and 8% of all twins). Twins can either be monozygotic or dizygotic. TYPES OF TWINS There are five variations of twinning that commonly occur. The three most common variations are all fraternal: (1) male-female twins are the most common result, at about 40% of all twins born; (2) female fraternal twins (sometimes called sororal twins); (3) male fraternal twins. The last two are identical: (4) female identical twins and (5) (least common) male identical twins. Male singletons are slightly, about 5%, more common than female singletons. 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 to exist. Polar body twinning would result in "half-identical" twins.1 Fraternal twins Fraternal twins (commonly known as "non-identical twins") usually occur when two Fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as '''dizygotic''' as well as "biovular" twins. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. Dizygotic twins, like any other siblings, have an ''extremely'' small chance of having the exact same chromosome profile. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal 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 fraternal twins are simply brothers and/or sisters who happen to have the same age. Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternal twinning ranges from 6 per thousand births in Japan (similar to the rate of identical twins) to 14 and more per thousand in some Africa n states. Fraternals are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. 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. Identical twins Identical twins occur when a single Egg is fertilized to form one Zygote ('''monozygotic''') which then divides into two separate Embryo s. Although their traits and physical appearances are not exactly the same due to environmental conditions both in and outside the womb, they do have identical DNA. This is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide,2 regardless of ethnic background. The two embryos develop into Fetus es 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 2 days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins, which occurs 20-30% of the time. Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di") twins. In about 1% of identical twins 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 60%, principally due to cord entanglement prior to 32 weeks gestation. Mono/di twins have about a 25% mortality due to twin-twin transfusion. Di/di twins have the lowest mortality risk at about 9%, although that is still significantly higher than that of singletons.Benirschike K, "Multiple Gestation" in Creasy RK, Resnik R, (eds.) Maternal-Fetal Medicine: Principles and Practice; 5th ed. (2004):55-62. Philedelphia: Saunders.. Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are usually the same sex. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other. Some biologists also believe in the possibilty of the egg splitting first, then the two conceived eggs are separately semenated, resulting in identical appearance, but different sex) Monozygotic twins generally look alike. Fine physical details such as Fingerprint s will differ. As they mature, identical twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of identical twins are half-siblings rather than cousins. It is estimated that there are around 10 million identical twins and triplets in the world. The likelihood of a single fertilisation resulting in identical twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world. This is in marked contrast to fraternal twinning which ranges from about 6 per thousand births in Japan (almost similar to the rate of identical twins, which is around 4-5) to 15 and more per thousand in some parts of IndiaOleszczuk JJ, Keith DM et al. Projection of Population-based twinning rates through the year 2100 . ''J Reprod Med''. 44(11), 1999. 913–921. (and up to 24 in the US, which might mainly be due to IVF, in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown. Studies have shown that identical twins reared in different environments share similar personality traits, mannerisms, job choices, attitudes, and interests. These findings add to the belief that many behaviors are derived from genes.
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