Anomalies peculiar to multiple gestations

Congenital anomalies occur twice as often in twin pregnancies as in singlet ons.(23) In addition to the large range of structural malformations that ar e described in singleton fetuses, multiple gestations are at risk for addit ional anomalies that are unique to the twinning process. These unique anoma lies include twin-twin transfusion syndrome (TTTS), a serious condition occ urring because of an imbalance in placental flow in monochorionic gestation s; twin reversed arterial perfusion (TRAP) syndrome, a condition occurring because of absence of a functional heart in one twin together with perfusio n of this fetus in a reverse direction through its umbilical artery; and co njoined twinning, an extremely rare event occurring because of delayed and incomplete splitting of the early embryo. This article reviews and describe s each of these unique abnormal conditions in detail.