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Multifetal pregnancies may result from two or more fertilization events, from a single fertilization followed by a splitting of the zygote, or both. In part because of infertility therapy, the twin birth rate in the United States was 3.2 percent in 2019. For higher-order multifetal births, the number peaked in 1998 at 0.2 percent of all births. Subsequent efforts in the reproductive endocrinology community to curb this rate have led to declines. Specifically, the rate of triplets or more declined by 55 percent from 1998 to 2019 (Martin, 2021).

Multifetal gestations remain problematic for both the mother and her fetuses. For the fetus, multifetal births accounted for 3 percent of all live births in 2013 but for 15 percent of all infant deaths. Specifically, the infant mortality rate for twins was more than four times the rate for singletons, and for triplets it was 12-fold higher (Matthews, 2015). Neonates from multifetal gestations also make up a disproportionate percentage of very-low-birthweight newborns in the United States (Martin, 2021). These two outcomes in singletons and twins from Parkland Hospital are shown in Table 48-1. Risks for congenital malformations and preterm birth are greater with multifetal gestation and discussed later.

TABLE 48-1Selected Outcomes in Singleton and Twin Pregnancies Delivered at Parkland Hospital from 1988 through 2021

The mother may also experience higher morbidity and mortality rates, and these rise with the number of fetuses (Society for Maternal-Fetal Medicine, 2019). In one study of more than 44,000 multifetal pregnancies, the risks for preeclampsia, postpartum hemorrhage, and maternal death were twofold higher than these rates in singleton gestations (Walker, 2004). Rates of placenta previa and placenta accreta spectrum are increased (Miller, 2021a). Moreover, compared with rates for singletons, the peripartum hysterectomy rate was threefold greater for twins and 24-fold higher for triplets or quadruplets in one study (Francois, 2005).


Dizygotic versus Monozygotic Twinning

Multifetal gestations are often described by zygosity, amnionicity, and chorionicity, which are the number of zygotes, amnions, and chorions, respectively. Twin fetuses most often result from fertilization of two separate ova, which yields dizygotic or fraternal twins. Less often, twins arise from a single fertilized ovum that then divides to create monozygotic or ...

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