Unbalanced vascular anastomoses — Placental intertwin vascular anastomoses play a key role in the pathophysiology of TTTS. Dichorionic twins lack vascular connections between the placentas, even when fused, and do not develop TTTS. In monochorionic twins (2/3 of all monozygotic twin gestations), post-delivery placental injection studies have identified four types of vascular connections: