Multiple pregnancy involves more than one embryo (fetus) in any one gestation. Two independent mechanisms may lead to multiple gestation: segmentation of a single fertile ovum (identical, monovular, or monozygotic) or fertilization of separate ova by different spermatozoa (fraternal or dizygotic) multiple pregnancy. In the development of twins (the most frequent higher-order gestation), monozygotism is constant ( –4/1000 deliveries), whereas dizygotism has certain predispositions. Dizygotic twinning is inherited as a recessive autosomal trait via the female descendants. The father’s being a twin has little influence on the rate of twinning in his offspring. . | MULTIPLE PREGNANCY DEFINITIONS ASSOCIATION INCIDENCES And importance Multiple pregnancy involves more than one embryo fetus in any one gestation. Two independent mechanisms may lead to multiple gestation segmentation of a single fertile ovum identical monovular or monozygotic or fertilization of separate ova by different spermatozoa fraternal or dizygotic multiple pregnancy. In the development of twins the most frequent higher-order gestation monozygotism is constant 4 1000 deliveries whereas dizygotism has certain predispositions. Dizygotic twinning is inherited as a recessive autosomal trait via the female descendants. The father s being a twin has little influence on the rate of twinning in his offspring. Race is of special importance blacks have the greatest incidence of dizygotic twins about 50 1000 births in Western Nigeria whites are intermediate and Asians have the fewest 1-2 1000 births in Japan . Other factors influencing dizygotism include greater maternal height or weight increasing maternal age peaks at 35 5 years and white mothers of blood group O or A. In developed countries two of the major causes of multiple gestation are cessation of oral contraception and artificial ovulation induction. The latter is of particular concern for higher-order multiple gestations triplets and above are increasingly common to 2-fold increase in developed countries as a result of assisted reproductive technologies ART . Although these pregnancies are not at significantly increased risk from the ART they are at exceptional risk for immature or premature delivery and other morbidity and mortality associated with higher-order multiple gestations. In the heterogeneous population of the United States slightly 30 of twins are monozygotic and nearly 70 are dizygotic Fig. 12-1 . In such a population a useful estimate of the natural frequency of multiple gestation is that twinning occurs 12 per 1000 births 1 88 . Each increase in birth number may then be estimated 367 .
