As noted earlier, the incidence of congenital malformations is appreciably higher in multifetal gestations compared with that in singleton pregnancies. In one survey-based study, the congenital malformation rate was 406 per 10,000 twins compared with 238 per 10,000 singletons (Glinianaia, 2008). The malformation rate in monochorionic twins was almost twice that of dichorionic twin gestations. This increase has been attributed to the higher incidence of structural defects in monozygotic twins. Indeed, one large population-based study between 1998 and 2010 found that twins had a 73-percent greater risk of congenital heart disease than singletons. The risk was substantially higher among monochorionic twins (Best, 2015). But, from a 30-year European registry of multifetal births, structural anomaly rates rose steadily from 2.16 percent in 1987 to 3.26 percent in 2007 (Boyle, 2013). Yet, during this time, the proportion of dizygotic twins grew by 30 percent, whereas the proportion of monozygotic twins remained stable. his higher risk of congenital malformations in dizygotic twins over time correlated with increased availability of AR. An increase in rates of birth defects related to ART has been reported repeatedly (Boulet, 2016; Talauliker, 2012).
Multifetal gestations are more likely to be low birthweight than singleton pregnancies due to restricted fetal growth and preterm delivery. From 1988 to 2012 at Parkland Hospital, data were collected from 357,205 singleton neonates without malformations and from 3714 normal twins who were both liveborn. Birthweights in twins closely paralleled those of singletons until 28 to 30 weeks' gestation. Thereafter, twin birthweights progressively lagged (Fig. 45-8). Beginning at 35 to 36 weeks' gestation, twin birthweights clearly diverge from those of singletons.
In general, the degree of growth restriction increases with fetal number. The caveat is that this assessment is based on growth curves established for singletons. Several authorities argue that fetal growth in twins is diferent from that of singleton pregnancies. And thus, abnormal growth should be diagnosed only when fetal size is less than expected for multietal gestation. Accordingly, twin and triplet growth curves have been developed (Kim, 2010; Odibo, 2013; Vora, 2006). At Parkland, we use the standards of birthweight in twin gestations stratified by placental chorionicity for identification of suspected fetal-growth restriction (Ananth, 1998).
FIGURE 45-8 Birthweight percentiles (25th to 75th) for 357,205 singleton neonates compared with the 50th birthweight percentile for 3714 twins, Parkland Hospital 1988-2012. Infants with major
FIGURE 45-9 Marked growth discordance in monochorionic twins. (Used with permission from Dr. Laura Greer.) he degree of growth restriction in monozygotic twins is likely to be greater than that in dizygotic pairs (Fig. 45-9). With monochorionic embryos, allocation of blastomeres may not be equal, vascular anastomoses within the placenta may cause unequal distribution of nutrients and oxygen, and discordant structural anomalies resulting from the twinning event itself may afect growth. For example, the quintuplets shown in Figure 45-10 represent three dizygotic and two monozygotic fetuses. When delivered at 31 weeks, the three neonates from separate ova weighed 1420, 1530, and 1440 g, whereas the two derived from the same ovum weighed 990 and 860 g.