Complications of monochorionic twins

Morbidity and mortality are significantly increased in twin gestations compared with singleton pregnancies [1]. Among twin pregnancies, the relative risk of complications depends on whether each fetus is attached to its own placenta (dichorionic [DC]) or must share a placenta (monochorionic [MC]). The relative increase in risk of MC compared with DC twin pregnancies is of a magnitude similar to that of twin compared with singleton pregnancies. MC twins have a higher prevalence of growth retardation and death compared with DC twins, and several unique and threatening syndromes occur only in MC gestations [2]. The high risks of MC twin gestations are largely related to the vascular anatomy of the shared placenta and the presence of intertwin vascular connections. These anastomoses are implicated in twin-twin transfusion syndrome (TTTS) and co-twin sequelae after intrauterine demise of one twin. Only MC twins can be monoamniotic. If the fetuses share a placenta and an amniotic cavity, they face a high risk of mortality [3,4]. The identification of a MC twin pregnancy has important obstetric implications, some of which influence pregnancy management and limit certain treatment options. The sonographic examination of all twin pregnancies should include a specific effort to determine chorionicity and amnionicity [5]. Before considering the sonographic features useful in the prediction of chorionicity and amnionicity, it is helpful to review the embryology of placentation in twin pregnancies.