The high risks of monochorionic (MC) twin gestations are largely related to the vascular anatomy of the shared placentae and the presence of intertwin vascular connections. These anastomoses are implicated in twin-twin transfusion syndrome and co-twin sequelae after intrauterine death of one twin. Twin-twin transfusion syndrome is the most common complication of MC twinning. Obstetrical sonography, with the use of Doppler techniques, can be used to assess these high-risk pregnancies and to interrogate the vascular connections in the shared placentae. The information obtained can improve our understanding of the pathophysiology of syndromes that complicate these pregnancies and may help inform obstetrical management.