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A multiple gestation occurs when >1 fetus is carried during a pregnancy.


As of 2017, the overall rate of twin births was 33.3 in 1000 live births, and the rate of triplet or other higher order multiple births was 101.6 in 100,000 live births in the United States. Between 1980 and 2009, the twinning rate climbed 76% (18.9–32.2 per 100 live births) and has slowly increased since. The rate of triplet births escalated more rapidly, increasing 400% during the 1980s and 1990s, with a peak in 1998. The rate of triplets and higher order multiple births has dropped 46% from the 1998 peak to present. 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. The incidence of monozygotic twinning is constant at 3 to 5 per 1000 pregnancies, whereas the rate for dizygotic twinning varies from 4 to 50 per 1000 pregnancies.


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 107–1).

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

    3. 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.

  2. 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.

  3. 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.

    2. 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 ...

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