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The most common form of human multiple birth is twins (two babies), but the typical order of multiple births in other placental species differs dramatically. Some species give birth to multiple offspring as a matter of course and the resulting group of offspring is called a litter.
Terminology
Terms used for the order of multiple births are largely derived from the Latin names for numbers.
Higher order multiplesHigh orders of multiple births (three or more offspring in one birth) may result in a combination of fraternal (genetically different) and identical (genetically identical) siblings. The latter are also called super twins. For example, a set of quadruplets may consist of two sets of identical twins. In such a case each child has one identical and two fraternal siblings. Identical triplets or quadruplets are very rare, as this means that the original fertilized egg split and then one of the resultant cells splits again (producing identical triplets) or, even more rarely, a further split occurs, producing identical quadruplets). Sometimes the original fertilized egg will split twice (to produce four embryos), and all four may survive and produce quadruplets, or one of the embryos may not survive and result in identical triplets. Human multiple birthsImage:TwinBoysInBathtub.jpg Fraternal twin brothers
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 pregnancies that started out with ten, eleven, twelve or fifteen fetuses, but no known instances of live births of such high multiples in a single pregnancy. Most of these pregnancies are the result of fertility medications and ART, 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 frequencyImage:TripletsGirls.jpg Identical triplet sisters 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 fertile eggs being produced in on ovulatory cycle). 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. It is theorized[citation needed] that this is due to the higher FSH that older women sometimes have as their ovaries respond more sluggishly to FSH stimulation. 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 be attributed to the impact of fertility treatments, such as in vitro fertilisation. Younger patients who undergo treatment with fertility medication containing artificial FSH, followed by intrauterine insemination, are at particularly at risk for multiple births of higher order. Certain factors appear to increase the likelihood that a woman will naturally conceive multiples. These factors include:
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. Better nutrition also increases the likelihood of multiple births. RisksRecent increases over the last few years in the number of multiple births have also provoked concern over the risks to the fetus and to the mother. The greater the number of babies in a single pregnancy, the more likely they are to have 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 per embryo transfer during IVF to one or two. 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. Cultural aspectsCertain cultures consider multiple births a portent of either good or evil. A Korean tradition stating that triplets are 'lucky' has led to the seizure of all sets of triplets born in North Korea, apparently out of leader Kim Jong-il's fear that one may overthrow him in the future.[1] In Ancient Rome, the legend of the twin brothers who founded the city (Romulus and Remus) made the birth of identical twin boys a blessing, while identical twin girls were seen as an unlucky burden. See also
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