Twins, triplets and other multiple births occur with to varying degrees in most animal species, although the term is most applicable to species in the Placentalia subclass. A multiple birth, for human beings, results when more than one human baby is born from a single pregnancy.
The most common form of human multiple birth is twins (two babies), but cases of triplets (three), quadruplets (four), quintuplets (five), sextuplets (six), septuplets (seven), and octuplets (eight) 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. There have been a few sets of nonuplets (nine) in which a few babies were born alive, and there have been pregnancies — but no known instances of live births — of decaplets (ten), undecaplets (eleven), duodecaplets (twelve), and quindecaplets (fifteen). Most of these are a result of fertility drugs, though a set of duodecaplets was conceived spontaneously (without the aid of fertility treatments) in Argentina in 1992.
There are two types of multiple births, fraternal and identical. Identical siblings arise where one egg is fertilised and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilisation and implantation of more than one egg, so fraternal siblings are not genetically identical.
Higher orders of multiple births (triplets or higher) may result in a combination of fraternal and identical siblings, and are occasionally given the term of super twins. As an example, a set of quadruplets may consist of two sets of identical twins. For simplicity, we'll say two are male, the other two, female. If the two male children have an identical genetic code, then they are considered identical twins in and of themselves. Likewise for the female children. The males are considered fraternal to the females, but identical to their brother. This isn't limited to just male/female quadruplets, but they statistically make up half of all quadruplet identical super twins. Other cases may involve just one set of identical twins with one or more fraternal siblings.
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 (several embryos are usually implanted to compensate for their lower viability).
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%. Much of the increase can probably attributed to the impact of fertility treatments, such as in vitro fertilisation. 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 race — African women have a frequency of 16 fraternal twin pairs per 1,000 births; East Asian women have a frequency of 3 pairs of fraternal twins per 1,000 births; Caucasian women have a frequency of 8 pairs of fraternal twins per 1,000 births
- 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 the increasing life expectancy for women have contributed to the rise in the rate of multiples over the last fifty years.
Recent increases over the last few years in the number of multiple births have also provoked concern over the risks to the fetus and also to the mother.
The greater the number of babies in a single pregnancy, the more likely they are to have a low birth weight, to be born prematurely and to consequently suffer medical problems. For example, in 1999, 53% of babies in multiple births were premature, compared to 7% of singletons.There is also a higher rate of stillbirths for multiples than for singletons.
Virtually all obstetrical risks are increased for the mother during a pregnancy with multiples. As many multiple pregnancies today are the result of the use of fertility therapy, efforts are being made to reduce the risks of multiple pregnancy, specifically triplets or more, by limiting the number of embryos embryo transfer during IVF to 1 or 2. Sometimes a selective reduction -- the abortion of one or more of the multiple fetuses -- is performed to give the pregnancy a higher chance of producing at least one healthy or live child.
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