Figure 1. The type of placenta in identical twins is determined by when the twinning process occurs in the first two weeks of pregnancy. Twinning within the first four days results in separate (dichorionic) placentas, after four days a shared (monochorionic) placenta with separate (diamnionic) bags of water, and after eight days the twins will share the placenta and water bag (monoamnionic). TTTS occurs in monochorionic twins.