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Twins, Triplets & More Information

  • A multiple birth occurs where more than one fetus exits the womb in a single pregnancy. Different names for multiple birth are used, depending on the actual multiple. Common multiples are two and three, known as twins and triplets respectively. Twins, triplets and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species...
  • There are two types of multiple births, fraternal (dizygotic) and identical (monozygotic). Identical siblings arise where one egg is fertilized and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilization and implantation of more than one egg, so fraternal siblings are not genetically identical, and instead have an analogous genetic similarity of any brothers and sisters.

    The most common form of human multiple birth is twins (two babies), but cases of higher orders up to octuplets (eight babies) have all been recorded with all siblings being born alive. The largest set in which all members survived more than a few days is septuplets, the first of which was in 1997. The largest set to have even a single member survive is octuplets, in 1998 (with the seven surviving octuplets born in Texas).

    There have been a few sets of nonuplets (nine) in which a few babies were born alive, though none lived longer than a few days. There have been cases of human pregnancy which started out with ten, eleven, twelve and fifteen fetuses, but no known instances of live births of such high multiples in a single pregnancy. Most of these pregnancies are a result of fertility medications, though a set of duodecaplets (twelve) was conceived spontaneously (without the aid of fertility treatments) in Argentina in 1992.

    Multiple pregnancies in humans are usually delivered before the full term of 40 weeks gestation. The average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

    Causes and frequency

    Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as IVF (several embryos are usually implanted to compensate for their lower viability) or fertility drugs (which can induce multiple ovulation).

    In general, twins occur naturally at approximately the rate of 1/89 of singleton births, triplets at 1/89 the rate of twin births, and so on (Hellin's Law). However, for reasons that are unknown, the older a woman is, the more likely she is to naturally have a multiple birth.

    The number of multiple births has increased over the last decades. For example, in Canada, between 1979 and 1999, the number of multiple birth babies increased 35%. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of.[citation needed] Much of the increase can probably attributed to the impact of fertility treatments, such as in vitro fertilization. Younger patients who undergo treatment with fertility medication containing FSH followed by intrauterine insemination are at particular risk for multiple births of higher order.

    Certain factors appear to increase the likelihood that a woman will naturally conceive multiples. These factors include:

    - mother's age: women over 35 are more likely to have multiples than younger women

    - mother's use of fertility drugs: approximately 35% of pregnancies arising through the use of fertility treatments such as IVF involve more than one child

    The increasing use of fertility drugs as well as the increasing life expectancy for women have contributed to the rise in the rate of multiples over the last fifty years.

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