The duration of gestation shortens with accruing fetal number. More than five of every 10 twins and nine of 10 triplets born in the United States in 2015 were delivered preterm (Martin, 2017). Prematurity is sixfold and tenfold greater in twins and triplets, respectively (Giufre, 2012). One review showed that approximately 60 percent of preterm births in twins are indicated, about a third result from spontaneous labor, and 10 percent follow prematurely ruptured membranes (Chauhan, 2010). In another analysis of almost 300,000 live births, the proportion of preterm birth associated with premature membrane rupture rose with gestational plurality from 13 percent with singletons to 20 percent with triplets or more (Pakrashi, 2013).
Although the causes of preterm delivery in twins and singletons may be diferent, neonatal outcome is generally the same at similar gestational ages (Kilpatrick, 1996; Ray, 2009; Salem, 2017). However, outcomes for pre term twins who are markedly discordant may not be comparable with those for singletons because whatever caused the discordance may have long-lasting efects (Yinon, 2005) .
Historically, twins have been considered cognitively delayed compared with singletons (Record, 1970; Ronalds, 2005). However, in cohort studies evaluating normal-birthweight term newborns, cognitive outcomes between twins and singletons are similar (Lorenz, 2012). Christensen and associates (2006) found similar national standardized test scores in the ninth grade in 341r1 twins and 7796 singletons born between 1986 and 1988.
In contrast, among normal-birthweight neonates, the cerebral palsy risk is higher among twins and higher-order multiples. For example, the cerebral palsy rate has been reported to be 2.3 per 1000 in singletons, 12.6 per 1000 in twins, and 44.8 per 1000 in triplets (Giufre, 2012). Greater risks of fetalgrowth restriction, congenital anomalies, twin-twin transfusion syndrome, and fetal demise of a cotwin are suggested contributors to these diferences (Lorenz, 2012).
Several unique complications arise in multifetal pregnancies. These are described in twins but can be found in higher-order multifetal gestations. Most fetal complications due to the twinning process itself are seen with monozygotic twins. heir pathogenesis is best understood after reviewing the possibilities shown in Figure 45-1.
Only about 1 percent of all monozygotic twin gestations will share an amnionic sac, and approximately 1 in 20 monochorionic twin gestations are monoamnionic (Hall, 2003; Lewi, 2013). Diamnionic twins can become monoamnionic if the dividing membrane spontaneous or iatrogenically ruptures. Their morbidity and mortality rates then mirror those of monoamnionic twins.
Historical mortali ty rates in monoamnionic twins were reported to be as high as 70 percent. Contemporary outcomes are improved, yet the demise rate after viability remains elevated (Post, 2015). Of those fetuses alive before 16 weeks' gestation, less than half survive until the neonatal period. Fetal abnormalities and spontaneous miscarriage contribute to most losses (Prefumo, 2015). After 20 weeks, the perinatal mortality rate for monoamnionic twin pregnancies approximates 15 percent (Shub, 2015). A high fetal death rate is attributable to preterm birth, congenital anomalies, twin-twin transfusion syndrome, or cord entanglement.