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Since antiquity, multiple births have fascinated mankind. Twins, triplets, and higher order births attract public attention and bemusement that often belies appreciation of specific management challenges not associated with singleton pregnancies. In addition, certain complications of pregnancy, such as preterm birth, occur more frequently with higher order gestations. Optimal management requires knowledge of issues associated with multifetal gestation during pregnancy, at delivery, and into the neonatal period.

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Multifetal gestations are classified on the basis of zygosity and chorionicity.1Dizygotic twins arise from independent fertilization of separate ova. Typically, two placentas form with distinct chorionic and amniotic membranes separating each fetus. Monozygotic twins develop from a single fertilized ovum that subsequently divides at or before the blastocyst stage of embryogenesis. The factors promoting fission of the conceptus are poorly understood. The timing of embryonic division determines whether the monozygotic twins will be separated by amnion and chorion (dichorionic, diamniotic), amnion only (monochorionic, diamniotic), or not separated (monoamniotic, monochorionic). Dizygotic twins account for about two thirds of spontaneous twin births and are almost always dichorionic, diamniotic. Monochorionic twins share a single placenta with a shared circulation. This developmental phenomenon carries a significant mortality risk and may lead to complications such as twin-twin transfusion syndrome (discussed under “Complications” in this chapter).2 Higher order multiples can be a combination of monozygotic and dizygotic gestations. Monozygotic triplets arising from a single fertilized ovum are extremely rare.

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The twin birth rate in 2005 was 32.2 per 1000 births, a 42% increase from 1990, and up 70% since 1980.3 The incidence in 2005 was unchanged from 2004. It is not clear whether this represents a true stabilization of a persistent upward trend over the previous 25 years. The triplet+ birth rate (the number of triplet, quadruplet, and higher order multiple births) increased dramatically during the 1990s, peaking at 193.5 per 100,000 births in 1998. Since then, the rate has slowly declined to 161.8 per 100,000 births in 2005. However, this rate is still more than fourfold higher than in 1981.

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The increase in twin and higher order multiple births has occurred in conjunction with increases in maternal age and the use of assisted reproductive technologies, ovulation-inducing agents, and artificial insemination. It is noteworthy to consider that nearly half of very-low-birth-weight multiples hospitalized in newborn intensive care units may involve pregnancies assisted by infertility treatments.4 In the late 1990s, the American Society of Reproductive Medicine recommended reducing the number of embryos transferred per cycle. This may explain the decline in triplet+ births since that time.5 The remarkable increase in multiple births is recognized as a significant public health concern because of the associated mortality and morbidity risks discussed next.

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Multifetal gestations exhibit certain complications also encountered during a singleton gestation. Of these, the single most significant is preterm delivery (see Table 46-1). Once delivered, the preterm product of a multifetal gestation is at risk of experiencing any ...

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