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Ask Dr. Jana

Fraternal vs. Identical Twins
QUESTION
Dear Dr. Jana,
What is the difference between fraternal twins and identical twins? Can twins be conceived by two different sperms? If so, the twins would be called what? Would the twins look alike or different?

— Missy

ANSWER
March 12, 2001
Dear Missy,
Quite simply, identical twins are the result of a single egg being fertilized by a single sperm. The single fertilized egg then divides into two separate embryos that go on to become identical twins.

Fraternal twins are the result of two separate eggs being ovulated at the same time. In this instance, two different sperm are responsible for fertilizing the two eggs.

One way to think of this is that fraternal twins are really just like any other siblings except for the fact that they happened to be conceived at the same time. They are as likely or unlikely to look like each other as any other siblings. Identical twins, however, generally look more alike, having originated from the same egg.

— by Laura Jana, M.D., F.A.A.P.

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