the placenta(s) of a twin pregnancy; if dizygotic, the placentas may be separate or fused, the latter retaining two amnionic and two chorionic sacs (dichorionic diamnionic placenta); if monozygotic, the placenta may be a monochorionic monoamnionic placenta or monochorionic diamnionic placenta, depending on the stage at which twinning took place; if twinning occurs early, there may be a fused placenta with two chorionic and two amnionic membranes.