Fraternal twins occur when two separate eggs are fertilized in the same cycle—often the result of ovarian stimulation or multiple embryo transfer during fertility treatment. Genetically, they're no more similar or different than any other siblings.
Identical twins, which account for one-third of twin births, come about when a single fertilized egg subsequently splits in two. Identical twins share the same genes, but are never truly identical.
Depending on when the embryo divides, identical twins can be completely separate, share a placenta, share a placenta and an amniotic sac, or be conjoined (Siamese).
Find out more about this book:YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy