It is of great importance to know the different types of twins and the words used to describe them.
Monozygotic twins result from the division of one fertilized egg, or zygote.
The majority, those for whom the division occurs between days five and seven, will share one placenta and will have separate amniotic sacs. In 2 percent of monozygotic twins the division occurs after day eight, in which case they will share one placenta, one chronic membrane, and one amniotic sac. Most of these who develop side by side have only a fifty-fifty chance of staying alive. These identical twins are always the same sex. They frequently look very much a like in appearance however there may be subtle differences between the two like a slight difference in hair color or a small birth mark.