These rates of multi fetal pregnancies have a direct efect 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). Speciically, 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
TABLE 45-1. Selected Outcomes in Singleton and Twin
Outcome Singletons (No.) Twins (No.)
Pregnancies 202,306 241n2
Birthsa 202,306 4824
Stillbirths 1 011 (5.0) 114 (23.6)
Neonatal deaths 590 (2.9) 92 (1n9.5)
Perinatal deaths 1601 (7.9) 206 (42.7)
Very low birthweight 1927 (9.6) 507 (1n07.6) aBirth data are represented as number (per 1000). bDenominator for neonatal deaths and very low birthweight is liveborn infants. Data from Dr. Don Mcintire.
for singletons! From Parkland Hospital, a comparison of singleton and twin outcomes is shown in Table
These risks are magnified further with higher-order births.
he mother may also experience higher obstetrical morbidity and mortality rates. hese rates lso rise with the number of fetuses (Mhyre, 2012; Young, 2012). In one study of more than 44,000 multifetal pregnancies, the risks for preeclampsia, postpartum hemorrhage, and maternal death were twofold higher than these rates in singleton gestations (Walker, 2004). The risk for peripartum hysterectomy is also greater. Francois and associates (2005) reported this to be threefold for twins and 24-fold for triplets or quadruplets. Last, compared with women with a singleton pregnancy, these mothers are at increased risk for depression as well as parental divorce (Choi, 2009; Jenna, 2011).
Twin fetuses usually result from fertilization of two separate ova, which yields dizygotic orratenal twins. Less often, twins arise from a single fertilized ovum that then divides to create monozygotic or identical twins. Either or both processes may be involved in the formation of higher numbers. Quadruplets, for example, may arise from as few as one to as many as four ova. These traditional models of twinning discussed in the next sections have been taught for more than 50 years and remain the widely accepted theory. More recently, Herranz (2015) ofered a provocative alternative hypothesis, which posits that monozygotic twinning occurs with splitting at the postzygotic twocell stage. Notably, data are not robust in support of either the traditional or the newly proposed model (Denker, 2015).
Dizygotic twins are not in a strict sense true twins because they result from the maturation and fertilization of two ova during a single ovulatory cycle. Moreover, from a genetic perspective, dizygotic twins are like any other pair of siblings.