Multifetal Pregnancy (Twins)

When two mature ova are produced in one ovarian cycle, both have a potential to be fertilized by separate sperm. This results in two zygotes or dizygotic twins. There are always two amnios, two chorions, and two placents that may be fused together. These dizygotic, or fraternal, twins may be the same sex or different sexes and are genetically no more alike than siblings born at different times. Dizygotic twinning occurs in families, more often amongafrican-american women that among Caucasian women and lest often among asian women. Dizygotic twinning increases in frequency with maternal age up to 35 years, with parity, and with the use of fertility drugs

Identical twins (monozygotic twins) develop from one fertilized ovum, which the divides. They are the same sex and have the same gonotyp. If division occurs soon after fertilization, two embryos, two amnions, two chorions, and two placentas that may be fused will develop. Most often, division occurs between 4 and 8 days after fertilization and there are two embryos, two amnions, one chorion, and one placenta. In this case are two embryos within a common and a comman chorion with one placenta. This often causes problems in circulation because the umbilical cord may tangle together, and one or both fetuses may die. If division occurs very late, cleavage may not be complete, and conjoined twins could result. Monozygotic twinning occurs in proximately 1 of 250 births (Cunningham et al., 1997).