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  1. Definition. A multiple gestation occurs when more than one fetus is carried during a pregnancy.

  2. Incidence. In 2005, the overall rate of twin births was 32.2 in 1000 live births and the rate of triplet births was 1.6 in 10,000 live births. The incidence of multiple gestation pregnancies is probably underestimated. Fewer than half of twin pregnancies diagnosed by ultrasonography during the first trimester are delivered as twins, a phenomenon that has been termed vanishing twin. Two gestational sacs can be identified with ultrasonography by 6 weeks' gestation. In addition, routine screening for maternal α-fetoprotein (AFP) may identify pregnancies with multiple gestations at an early gestational age. About a third of twins in the United States are monozygotic. Between 1980 and 1994, there was a 42% increase in the number of twin births in the United States. The rate of triplet births escalated more rapidly, increasing 100% between 1980 and 1989. By 2001, the rate of higher-order multiples had leveled off while the rate of twin births continued to increase. The incidence of monozygotic twinning is remarkably constant at 3–5 per 1000 pregnancies, whereas the rate for dizygotic twinning varies from 4–50 per 1000 pregnancies.

  3. Pathophysiology. Placental classification and determination of zygosity are important in the pathophysiology of twins.

      1. Classification. Placental examination affords a unique opportunity to identify two thirds to three fourths of monozygotic twins at birth.

          1. Twin placentation is classified according to the placental disk (single, fused, or separate), number of chorions (monochorionic or dichorionic), and number of amnions (monoamniotic or diamniotic) (Figure 103–1).

          1. Heterosexual (assuredly dizygotic) twins always have a dichorionic placenta.

          1. Monochorionic twins are always of the same sex. All monochorionic twins are believed to be monozygotic. In 70% of monozygotic twin pregnancies, the placentas are monochorionic, and the possibility exists for commingling of the fetal circulations. Less than 1% of twin pregnancies are monoamniotic.

      1. Placental complications. Twin gestations are associated with an increased frequency of anomalies of the placenta and adnexa; for example, a single umbilical artery or velamentous or marginal cord insertion (6–9 times more common with twin gestation). The cord is more susceptible to trauma from twisting. The vessels near the insertion are often unprotected by Wharton jelly and are especially prone to thrombosis when compression or twisting occurs. Intrapartum fetal distress from cord compression and fetal hemorrhage from associated vasa previa are potential problems with velamentous insertion of the cord.

      1. Determination of zygosity. The most efficient way to identify zygosity is as follows:

          1. Gender examination. Male-female pairs are dizygotic. The dichorionic placenta may be separate or fused.

          1. Placental examination. Twins with a monochorionic placenta (monoamniotic or diamniotic) are monozygotic. Care should be taken not to confuse apposed fused placentas for a single chorion. If doubt exists on a gross inspection of the dividing membranes, a transverse section should be studied. The zygosity of twins of the same sex with dichorionic membranes cannot be immediately known. Genetic studies are needed (eg, blood typing, human leukocyte antigen typing, DNA ...

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