Multiple Gestation: Fetal and Maternal Considerations

Over the past three decades, there is a worldwide increase of multifetal gestations, which is mostly attributed to advances in artificial reproductive technology. These pregnancies have a major contribution to neonatal morbidity and mortality, including high rates of preterm deliveries, very low birth weight infants, and congenital anomalies. Acknowledgment of these risks over the past few years led to changes in infertility treatment practices, contributing to a steady decline in multiple gestation rates. Classification of twin pregnancies includes the zygosity (i.e., genetic formation of the fetuses), chorionicity, which refers to the number of outer membranes and placentas, and amnionicity, referring to the number of separate amniotic sacs. Because of the high rate of early complications, first trimester assessment of chorionicity and amnionicity, and screening for fetal anomalies are crucial. This may enable close follow-up and discussion of selective feticide when indicated. Twins are at an increased risk of congenital anomalies, with monozygotic twins at highest risk of birth defects compared with both singletons and dizygotic twins. Monochorionic pregnancies have a shared blood system, because of communicating vessels over their single placenta, and may develop unique complications including twin-twin transfusion syndrome, selective intrauterine growth restriction, twin anemia polycythemia sequence, and twin reverse arterial perfusion syndrome. Half of complicated monochorionic pregnancies require intrauterine interventions. Long-term outcome of twins is mostly dependent on gestational age and perinatal complications.