Chapter 13: SONOGRAPHY IN MULTIPLE GESTATION

INTRODUCTION

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.

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 defects^7,8 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