In single-ovum twins, there is always a certain area of the placenta in which there is anastomosis between vascular systems which is never present in the fused placenta of double-ovum twins. Thus, if at an early period the heart of one embryo is considerably stronger than that of the other, a gradually increasing area of the communicating portion of the placenta is monopolized by the former, so that its heart increases rapidly in size, whilst that of the latter receives less blood and eventually atrophies.
—J. Whitridge Williams (1903)
In Williams’ time, a great deal concerning the embryological and morphological development of multifetal pregnancies was unknown. These pregnancies may result from two or more fertilization events, from a single fertilization followed by a splitting of the zygote, or from a combination of both. Multifetal gestations were problematic during those times and remain so today for both the mother and her fetuses. For example, in this country, approximately a fourth of very-low-birthweight neonates—those born weighing <1500 g—are from multifetal gestations (Martin, 2017).
Fueled largely by infertility therapy, both the rate and the number of twins and higher-order multifetal births grew dramatically during the 1980s and 1990s in the United States. National data from Martin and coworkers (2017) presented here is informative. The twinning rate rose 76 percent from 18.9 per 1000 live births in 1980 to 33.2 in 2009. During the same time, the number of higher-order multifetal births peaked in 1998 at a rate of 1.9 per 1000 total births. Since then, however, evolving infertility management has lowered rates of higher-order multifetal births—especially among non-Hispanic white women. For example, the rate of triplets or more declined by more than 50 percent from 1998 to 2015 in this demographic group. And, in 2015, the overall multifetal birth rate was 34.5 per 1000, with twins representing nearly 97 percent of these births.
These rates of multifetal pregnancies have a direct effect on the rates of preterm birth and its comorbidities. In addition, the risks for congenital malformation and its consequences are greater with multifetal gestations. Importantly, this increased risk applies to each fetus and is not simply the result of more fetuses. In sum, in 2013 in the United States, multifetal births accounted for 3 percent of all live births but for 15 percent of all infant deaths. Moreover, the risk of infant death rose proportionally with the number of fetuses in the pregnancy (Matthews, 2015). Specifically, the infant mortality rate for twins was more than four times the rate for single births. In the same year, the infant mortality rate for triplets was nearly 12 times the rate for singletons, and for quadruplets, it was a staggering 26 times that for singletons! From Parkland Hospital, a comparison of singleton and twin outcomes is shown in Table 45-1. These risks are magnified further with higher-order births.