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Aristotle long ago aptly noted that multiple births are “praeter naturam,” that is, “outside nature’s normal course.” Being outside the normal, their reception in society is also different than that afforded to the single child. In many parts of the world, they are seen as good luck and have become absorbed into the local mythology. This was the case in ancient Rome and Greece, for example, and also in Mesoamerica. In other parts of the world, as was often the case in some but not all parts of Africa, they were not welcomed, being seen as evidence of maternal marital infidelity.
The fact that Aristotle’s dictum remains correct centuries later is based on two inescapable facts: First, all multiple births are high-risk pregnancies in contrast to singletons (single births), and second, as the human female is genetically programmed to have a single child, both mother and child suffer these risks. This latter statement relates to the facts that under normal circumstances 98 percent of human pregnancies deliver one child only and that the distensibility of the human uterus is clearly limited. This reality is probably the greatest reason that higher-order multiples deliver preterm.
Twins are the most common form of multiple pregnancies, followed by triplets and then the rare occurrences of quadruplets and quintuplets. Prior to modern technologies for assisted reproduction, the incidence of twins was 1 in 89 deliveries, of triplets 1 in 892, and of quadruplets 1 in 893. All that changed after 1975 when clomiphene citrate became available; even more drastic changes followed post-1985 with the introduction of in vitro fertilization and embryo transfer. The rate of twin births has more than doubled, while that of triplets has tripled. U.S. trends were duplicated in many other localities that have access to modern treatments for infertility.
Twins are either identical or fraternal, though neither term is correct in that identical twins, derived from a single fertilized ovum, are never truly identical at the molecular genetic level, and fraternal twins, derived from two fertilized ova, may be female as well as male. More precise terms are monozygotic (MZ), meaning “one-egg,” and dizygotic (DZ), meaning “two-egg,” which both refer to embryological origins. Natural twins, the most frequent form of multiples, occur most commonly in Africans and least commonly in Asians, with the Euro-American Caucasian population in-between. The exact reasons for this have never been clarified, but it is known that the majority of twins in African populations are dizygotic, whereas the majority in Asian populations are monozygotic. In Caucasians, approximately two-thirds of naturally occurring twins are DZ and one third are MZ. In Africans, DZs far outnumber the MZs, presumably because of frequent double ovulation. In Asians, MZs predominate due to the relative scarcity of double ovulation. These racial variations have always been regarded as fact within the literature on multiples and have never been interpreted as signs of racial superiority/inferiority.
A third type of twins is the so-called Siamese or conjoined variety, named after the twins Eng and Chang, from Siam (now Thailand). These occur in somewhere between 1 in 50,000 and 1 in 100,000 births. Their cause is due to partial division of one zygote after thirteen days post fertilization. Female conjoined twins are more common than males in a ratio of 1.6:1, for reasons that are not entirely clear. It has been suggested that when conjoined twinning occurs at the embryonic plate, it is more lethal in males, though this has not been proven.
The prime cause of morbidity and mortality among all multiples is preterm delivery and low birth weight, which go hand in hand. Whereas the normal gestational length for singletons is forty weeks, the median gestational length is thirty-seven weeks for twins, thirty-three weeks for triplets, thirty-one for quadruplets, and twenty-nine for quintuplets. The increased numbers of preterm multiples raises U.S. rates of cerebral palsy by at least 7 percent. Mothers of multiples face five risks: anemia, postpartum bleeding, PIH (high blood pressure), polyhydraminos (excess water in one of the amniotic sacs), and preterm labor.
- Blickstein, Isaac, and Louis Keith, eds. 2001. Iatrogenic Multiple Pregnancy. London: Parthenon.
- Blickstein, Isaac, and Louis Keith, eds. 2005. Multiple Pregnancy: Epidemiology, Gestation, and Perinatal Outcome. 2nd ed. London: Parthenon.
- Keith, Louis , and Isaac Blickstein, eds. 2002. Triplet Pregnancies and Their Consequences. London: Parthenon.
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