Amniocity refers to membranes, chorionicity referes to placentation. All monochorionic (shared placenta) twins are monozygotic but not all dichorionic twins are dizygotic. So a shared placenta doesn't confound things but a fused placenta does. My twins had a placenta that was unable to be differentiated as fused or shared on gross inspection. Semi identical twins adds other confounding thought but the incidence is going to be very low.