In the 2003 First Aid on p. 82 it says that monozygotic twins have 1 placenta, 1 chorion, and 2 amniotic sacs. Then it says that monozygotic twins develop 2 placentas (separate/fused), 2 chorions, and 2 amniotic sacs. Which is it supposed to be?
My peripheral read on this subject suggests that monozygotic twins can have either of those scenarios depending on when the splitting of the placenta occurs. 70% have monochorionic, diamniotic placentas, and the other 30% have dichorionic, diamniotic placentas.