In humans, as the number of fetuses per pregnancy rises, the percentage of male conceptuses declines. Strandskov and coworkers (1946) found the percentage of males in 31 million singleton births in the United States was 51.6 percent. For twins, it was 50.9 percent; for triplets, 49.5 percent; and for quadruplets, 46.5 percent. Swedish birth data spanning 135 years reveals the number of males per 100 female newborns was 106 among singletons, 103 among twins, and 99 among triplets (Fellman, 2010). Females predominate even more in twins from late twinning events. For example, 68 percent of thoracopagus conjoined twins are female (Mutchinick, 2011). Two explanations have been ofered. First, beginning in utero and extending throughout the life cycle, mortality rates are lower in females. Second, female zygotes have a greater tendency to divide.
Twins of opposite sex are almost always dizygotic. In rare instances, due to somatic mutations or chromosome aberrations, the karyotype or phenotype of a monozygotic twin gestation can be diferent (Turpin, 1961). Most reported cases describe postzygotic loss of the Y chromosome in one 46,XY twin resulting in a phenotypically female twin with Turner syndrome (45,X). Zech and coworkers (2008) found a rare case of a 47,Y zygote that underwent postzygotic loss of the X chromosome in some cells and loss of the Y chromosome in other cells. The phenotype of the resultant twins was one male and one female. Karyotype analyses revealed both to be 46,XXJ46,XY genetic mosaics.
he risk for twin-speciic complications varies in relation to both zygosity and chorionicity-the number of chorions. Shown in Table 45-2, the latter is the more important determinant. Speciically, perinatal mortality and neurological injury rates are greater in monochorionic diamnionic twins compared with dichorionic diamnionic pairs (Hack, 2008; Lee, 2008). In one retrospective analysis of more than 2000 twins, the risk of fetal demise in one or both monochorionic twin(s) was twice that in dichorionic multifetal gestations (McPherson, 2012). Moreover, the prospective risk of antepartum stillbirth is higher for monochorionic than for dichorionic twins at all preterm gestational ages. he highest risk is before 28 weeks' gestation (Glinianaia, 2011). In contrast, chorionicity diferences do not signiicantly afect maternal outcomes (Carter, 2015).
This has become an integral tool to assist in multifetal pregnancy management. Indeed, the diagnosis and evaluation of a multifetal gestation is now considered a recognized indication for irst-trimester sonography (Reddy, 2014). In addition, the North American Fetal Therapy Network (NAFTNet)-a
TABLE 45-2. Overview of the Incidence of Twin Pregnancy Zygosity and Corresponding Twin-Specific Complications
Rates of Twin-Specific Complications in Percent consortium of 30 medical institutions in the United States and Canada-have provided recommendations for determination of chorionicity using sonography (Emery, 2015).