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| |    Twins: Identical vs. Fraternal by Elisa Ross, MD reviewed and revised by Marjorie Greenfield, M.D. For most expectant parents, the news that twins are on the way usually evokes a mixture of happy anticipation and apprehension. The parents are sure to have many questions, including speculation as to whether the pair will be identical or fraternal. Identical twins In the United States today, about one-third of twins are identical. These pairs, with their breathtaking similarities, come from the same egg and sperm. After the egg and sperm have joined, the resultant cell, called a zygote, starts to multiply into the millions of diverse cells that eventually compose a single baby. In identical twins, however, the resultant cell separates into two zygotes, each with the same genetic material. This kind of twinning happens in about one out of every 240 pregnancies. It takes place among all families and cultures and is not caused by fertility drugs. Fraternal twins On the other hand, fraternal twins are the result of two different eggs becoming fertilized by two different sperm. Although both zygotes share the same uterus, they are no more similar than any other set of siblings. This kind of twinning runs in families and is prevalent in certain ethnicities. Africa sees the highest incidence of fraternal twins; Asia, the lowest. Fertility drugs increase your chances of having this type of pair. When's the soonest I'll know? Most of the time, parents have to wait until birth to find out if their twins are identical or fraternal. Occasionally hints surface before then. For example, if the twins share the same amniotic sac, they are definitely identical. However, the majority of identical twins--and all fraternal twins--have their own amniotic sac, so you still may end up being in the dark until the delivery. And, of course, if ultrasound determines one twin to be a girl and one to be a boy, you can be certain they are fraternal. More information:
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