Anomalies peculiar to multiple gestations.

Congenital anomalies occur twice as often in twin pregnancies as in singletons. In addition to the large range of structural malformations that are described in singleton fetuses, multiple gestations are at risk for additional anomalies that are unique to the twinning process. These unique anomalies include twin-twin transfusion syndrome, twin-reversed-arterial-perfusion syndrome, and conjoined twinning. This article reviews and describes each of these unique abnormal conditions in detail.