A pair of twins created from a single fertilized egg. They usually share one placenta, one chorion, and two amniotic sacs, and in rare cases they share one of each (see figure below). Like dizygotic twins, their monozygotic counterparts can sometimes have fused placentas or two separate placentas. In the latter case, the twin-to-twin transfusion syndrome does not occur, which is a disease of the placenta affecting the pregnancies of monozygotic twins due to the fact they have a common monochorionic placenta (in some instances, there may be unequal sharing of nutrients and oxygen supplied by the placenta, leading to one twin becoming growth restricted).
