A. At 0 to 4 days postfertilization, an early conceptus may divide into two. Division at this early stage creates two chorions and two amnions (dichorionic, diamnionic). Placentas may be separate or fused. B. Division between 4 to 8 days leads to formation of a blastocyst with two separate embryoblasts (inner cell masses). Each embryoblast will form its own amnion within a shared chorion (monochorionic, diamnionic). C. Between 8 and 12 days, the amnion and amnionic cavity form above the germinal disc. Embryonic division leads to two embryos with a shared amnion and shared chorion (monochorionic, monoamnionic). D. Difering theories explain conjoined twin development. One describes an incomplete splitting of one embryo into two. The other describes fusion of a portion of one embryo from a monozygotic pair onto the other.
unequal growth and development of twin fetuses with the same setting of paternity lawsuits (Girela, 1997). Given that superfegestational age. cundation may also occur with ART, women should be advised
Supeecundation refers to fertilization of two ova within the to consider avoiding intercourse after embryo transfer (McNasame menstrual cycle but not at the same coitus, nor necesmara, 2016; Peigne, 201r1). sarily by sperm from the same male. An instance of superfecundation or heteropaternity, documented by Harris (1982), is demonstrated in Figure 45-2. he mother was delivered of • Factors Afecting Twinning a black neonate whose blood type was A and a white neonate Dizygotic twinning is much more common than monozygotic whose blood type was O. The blood type of the mother and her splitting of a single oocyte, and its incidence is influenced by husband was O. More recent cases have been reported in the race, heredity, maternal age, parity, and, especially, fertility )c::0)�000 5.)�:
FIGURE 45-2 An example ofdizygotic twin boys as the consequence of superfecundation.
treatment. By contrast, the frequency of monozygotic twin births is relatively constant worldwide-approximately 1 set per 250 births, and this incidence is generally independent of demographic factors. One exception is that rates of zygotic splitting are increased following ART (Aston, 2008).
Among diferent races and ethnic groups, the frequency of multifetal births varies significantly. In one analysis of more than 8 million births in the United States between 2004 and 2008, the rate oftwinning was 3.5 percent in black women and 3 percent in whites (Abel, 2012). Hispanic, Asian, and Native merican women had comparatively lowerrates than white women. In one rural community in Nigeria, twinning occurred once in every 20 births (nox, 1960)! hese marked diferences in twinning frequency may be the consequence ofracial variations in levels of follicle-stimulating hormone-FSH (Nylander, 1973).