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  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.


Multiple pregnancies represent about 1.2% of all pregnancies. 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 13-1). Environmental and dietary factors, seasonality, and family clustering considerably influence the twinning rate.

Figure 13-1.

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

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In Western countries about 30% of these pregnancies are iatrogenic (in a hypothetical obstetric population of 10,000 newborns, 84 spontaneous twins and 36 resulting from assisted reproduction), from which 27 twins would be spontaneously monozygotic and 2 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.1,2, and 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).2,3,4, and 5


The growing concern with multiple pregnancies is their higher mortality and greater incidence of adverse perinatal outcome compared to singleton pregnancies,6 to the doubling of risk for structural defects7,8 and to the higher risk of chromosomal anomalies.9 Although multiple pregnancies represent 1.2% 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 (Figure 13-2A and B).6 The perinatal mortality of monochorionic twins is 26 per 100, which remains 3 to 5 times higher than in dichorionic (DC) pregnancies (9 per 100). The rate of perinatal loss before 24 weeks in monochorionic compared with DC pregnancies is 12.2% versus 1.8%.6

Figure 13-2.

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 4 to 11 times higher, stillbirth is 3 to 13 times ...

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