Sonographically, a normal-appearing twin is accompanied by its cotwin, which is a large placenta containing multiple small anechoic cysts (Fig. 20-4, p. 391). Often, these pregnancies are terminated, but pregnancy continuation is increasingly adopted. First, the pregnancy prognosis is not as poor as previously thought, and live birth rates range between 20 and 40 percent (Dolapcioglu, 2009; McNamara, 2016). Second, the risk of persistent trophoblastic disease is similar whether the pregnancy is terminated or not (Massardier, 2009; Sebire, 2002). That said, given the limited number of cases, robust data for irm recommendations are lacking. Importantly, complications of expectant management include vaginal bleeding, hyperemesis gravidarum, thyrotoxicosis, and early-onset preeclampsia (McNamara, 2016). Many of these complications result in preterm birth with its attendant adverse perinatal sequelae as well as perinatal loss. Logically, close surveillance is needed for those continuing the pregnancy.
Fetal size inequality develops in approximately 15 percent of twin gestations and may relect pathological growth restriction in one fetus (Lewi, 2013; Miller, 2012). Generally, as the weight diference within a twin pair rises, the perinatal mortality rate increases proportionately. If it develops, restricted growth of one twin fetus, often termed selective etal-growth restriction, usually develops late in the second and early third trimester. Earlier discordancy indicates higher risk for fetal demise in the smaller twin. Speciically, when discordant growth is identified before 20 weeks, fetal death occurs in approximately 20 percent of the growth-restricted fetuses (Lewi, 2013).
he cause of birthweight inequality in twin fetuses is often unclear, but the etiology in monochorionic twins likely difers from that in dichorionic twins. Because the single placenta is not always equally shared in monochorionic twins, these twins have greater rates of discordant growth outside of TTTS than dichorionic twins. Discordancy in monochorionic twins is usually attributed to placental vascular anastomoses that cause hemodynamic imbalance between the twins. Reduced pressure and perfusion of the donor twin can cause diminished placental and fetal growth. Even so, unequal placental sharing is probably the most important determinant of discordant growth in monochorionic twins (Lewi, 2013). Occasionally, monochorionic twins are discordant in size because they are discordant for structural anomalies.
Discordancy in dichorionic twins may result from various factors. Dizygotic fetuses may have diferent genetic growth potential, especially if they are of opposite genders. Second, because the placentas are separate and require more implantation space, one placenta might have a suboptimal implantation site. Bagchi and associates (2006) observed that the incidence of severe discordancy is twice as great in triplets as it is in twins. This finding lends credence to the view that in utero crowding is a factor in multifetal growth restriction. Placental pathology may playra role as well. In one study of 668 twin placentas, a strong relationship between histological placental abnormalities and birthweight discordancy was observed in dichorionic, but not monochorionic, twin pregnancies (Kent, 2012).