In the third trimester, the larger fetal mass leads to accelerated placental maturation and relative placental insuiciency. In dizygotic pregnancies, marked size discordancy usually results from unequal placentation, with one placental site receiving more perfusion than the other. Size diferences may also reflect diferent genetic fetal-growth potentials. Discordancy can also result from fetal malformations, genetic syndromes, infection, or umbilical cord abnormalities such as velamentous insertion ' marginal insertion, or vasa previa (Chap. 44, p. 849).
Pregnancy-related hypertensive disorders are more likely to develop with multifetal gestations. The exact incidence attributable to twin pregnancy is diicult to determine because these gestations are more likely to deliver preterm and before preeclampsia usually develops. Also, women with twin pregnancies are often older and multiparous, qualities associated with lower rates of preeclampsia (Francisco, 2017). The incidence of pregnancy-related hypertension in women with twins is 20 percent at Parkland Hospital. In their analysis of 513 twin malformations, pregnancies complicated by stillbirth, and twin gestations with >25 percent discordance were also excluded. (Data from Dr. Don Mcintire.) pregnancies, Fox and coworkers (2014) identified 15 percent of parturients with preeclampsia. Another study compared 257 women with twins and gestational diabetes against 277 nondiabetic women carrying twins. hese researchers found a twofold greater risk of preeclampsia in women diagnosed with gestational diabetes (Gonzalez, 2012). Conversely, no specific zygosity confers a greater rate of hypertensive disorder in twin pregnancies (Lucovnik, 2016). Finally, from the National Center for Health Statistics, Luke and associates (2008) analyzed 316,696 twin, 12,193 triplet, and 778 quadruplet pregnancies. hese investigators noted that the risk for pregnancy-associated hypertension was significantly increased for triplets and quadruplets (11 and 12 percent, respectively) compared with that for twins (8 percent).
FIGURE 45-10 Davis quintuplets at 3 weeks following delivery. The first, second, and fourth newborns from the left each arose from separate ova, whereas the third and fifth neonates are from the same ovum.
hese data suggest that fetal number and placental mass are involved in preeclampsia pathogenesis. Women with twin pregnancies have levels of antiangiogenic soluble fms-like tyrosine kinase-1 (sFlt-1) that are twice that of singletons. Levels are seemingly related to greater placental mass rather than primary placental pathology (Bdolah, 2008; Maynard, 2008). Rana and coworkers (2012) measured antiangiogenic sFlt-1 and proangiogenic placental growth factor (PIGF) in 79 women with twins referred for evaluation of preeclampsia. In the 58 women identiied with either gestational hypertension or preeclampsia, there was an incremental rise in sFlt-1 concentrations, decline in PIGF levels, and increase in sFlt-lIPIGF ratios compared with normotensive twin pregnancies. With multifetal gestation, hypertension not only develops more often but also tends to develop earlier and be more severe. In the analysis of angiogenic factors mentioned above, more than half of afected women presented before 34 weeks, and their sFlt-lIPIGF ratio rise was more striking (Rana, 2012). This relationship is discussed in Chapter 40 (p. 716).