On the other hand, monozygotic or identical twins, although they have virtually the same genetic heritage, are usually not identical. Namely, the division of one fertilized zygote into two does not necessarily result in equal sharing of protoplasmic material. Monozygotic twins may actually be discordant for genetic mutations because of a postzygotic mutation, or may have the same genetic disease but with marked variability in expression. In female fetuses, skewed lyonization can produce diferential expression of X-linked traits or diseases. Further, the process of monozygotic twinning is in a sense a teratogenic event, and monozygotic twins have a higher incidence of often discordant malformations (Glinianaia, 2008). For example, in one study of 926 monozygotic twins, the prevalence of congenital heart defects was 12-fold greater than the general population rate, but 68 percent of afected infants had a normal sibling (Pettit, 20l3). From any of these mechanisms, dizygotic twins of the same sex may appear more nearly identical at birth than monozygotic twins.
• Genesis of Monozygotic Twins
The developmental mechanisms underlying monozygotic twinning are poorly understood. The incidence of monozygotic twins is increased two-to fivefold in pregnancies conceived using assisted reproductive technology (ART). he predisposition to splitting may stem from specimen handling, growth media, or sperm DNA microinjection or may arise from intrinsic abnormalities associated with infertility (McNamara, 2016).
The outcome of the monozygotic twinning process depends on when division oCCuts. If zygotes divide within the irst 72 hours after fertilization, two embryos, two amnions, and two chorions develop, and a diamnionic, dichorionic twin pregnancy evolves (Fig. 45-1). Two distinct placentas or a single, fused placenta may develop. If division occurs between the fourth and eighth day, a diamnionic, monochorionic twin pregnancy results. By approximately 8 days after fertilization, the chorion and the amnion have already diferentiated, and division results in two embryos within a common amnionic sac, that is, a monoamnionic, monochorionic twin pregnancy. Conjoined twins result if twinning is initiated later.
It has long been accepted that monochorionicity incontrovertibly indicated monozygosity. Rarely, however, monochorionic twins may in fact be dizygotic (Hackmon, 2009). Mechanisms for this are speculative, but in one review of 14 cases, nearly all had been conceived after ART procedures (Ekelund, 2008) . McNamara and colleagues (2016) ofer an excellent review of the mechanisms and evidence for both typical and atypical twinning.
In supeetation, an interval as long as or longer than a menstrual cycle intervenes between fertilizations. Superfetation requires ovulation and fertilization during the course of an established pregnancy, which is theoretically possible until the uterine cavity is obliterated by fusion of the decidua capsularis to the decidua parietalis. Although known to occur in mares, superfetation is not known to occur spontaneously in humans. Lantieri and associates (2010) reported a case ater ovarian hyperstimulation and intrauterine insemination in the presence of an undiagnosed tubal pregnancy. Most authorities believe that alleged cases of human superfetation result from markedly
FIGURE 45-1 Mechanism of monozygotic twinning. Black boxing and blue arrows in columns A, B, and C indicates timing of division.