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Key Terms

  1. Monozygotic twins: a pregnancy of 2 fetuses derived from the same zygote that divided between the day of fertilization and the 14th day. They can have phenotypic and genotypic differences and can have either a dichorionic or a monochorionic placentation.

  2. Dizygotic twins: a pregnancy of 2 fetuses derived from 2 different zygotes, resulting from the fertilization of 2 oocytes from the same cycle. They always have a dichorionic placentation.

  3. Chorionicity: type of placentation in cases of multiple pregnancy defined by the number of chorions.

  4. Lambda sign: sonographic sign resulting from the juxtaposition of 2 layers of amnion and 2 layers of chorion in a dichorionic twin pregnancy.


Twin pregnancies represent about 3% to 4% of all pregnancies.1 Large regional and racial differences with up to 15-fold variation in the prevalence of twinning at birth have been noted for many years (Figure 15-1). Environmental and dietary factors, seasonality, and family clustering considerably influence the twinning rate.

Figure 15-1.

The incidence of DZ twins differs in different populations. From left to right: Japan, European American population, African American population, and Nigeria.

In Western countries about 35% of these pregnancies are iatrogenic.1 Increased use of ovulation induction and assisted reproduction techniques (ART), coupled with delay in the reproductive age of pregnant women, have contributed to an increase in multiple pregnancies.1-3 These iatrogenic pregnancies contributed not only to the increase in the rate of dizygotic (DZ) twins but also of monozygotic (MZ) twin pregnancies.2,3 In iatrogenic pregnancies the ratio is altered and MZ twin pregnancies are more prevalent (6-fold increase).2-5

The growing concern with multiple pregnancies is their higher mortality and greater incidence of adverse perinatal outcome compared to singleton pregnancies, mainly from increased incidences of prematurity, fetal growth lag, structural malformations and chromosomal abnormalities.6-9

Although multiple pregnancies represent 3% to 4% of the population, they contribute to 10% to 14% of the overall perinatal mortality, a rate 5 to 10 times higher than that of singletons (Figures 15-2A and B). The perinatal mortality rate in monochorionic (MC) is twice that seen in dichorionic (DC) pregnancies, 12% versus 5%, (P < 0.001).9 The increase is primarily due to the fetal loss rates prior to 24 weeks, which are significantly higher in MC twins (60 per 1000) compared to DC twins (7 per 1000).6 Aside from the increase of preterm delivery and low birthweight when compared to DC twins and singleton pregnancies, MC twin pregnancies have higher rates of fetal malformations, discordance in fetal growth, and long-term neurologic morbidity.7-9

Figure 15-2.

A: The excess mortality in twins is predominantly due to the contribution of monochorionic twins. B: Compared with singletons, the rate of mortality of twins is ...

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