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Can an ultrasound distinguish fraternal twins from identical ones?
Q: I’m pregnant with twins and am wondering if my doctor will be able to tell me (via an ultrasound) if the twins will be fraternal or identical? I’d really like to know this!
Laura E. Stachel, M.D.
A: Fraternal twins account for about two-thirds of twin pregnancies in the United States. They develop from two separate eggs that were fertilized by two separate sperm. Genetically, they have as much in common as any other siblings with the same parents. Half of fraternal twins are same sex pairings (girl-girl or boy-boy) and half are mixed sex pairings (boy-girl). Identical twins, on the other hand, develop from a single egg and are always the same sex. These twins share the same genetic maternal.

Sometimes the ultrasound exam can give a clear answer to your question. If your twins are different sexes, then they must be fraternal! Also, twins that share the same amniotic sac and placenta are always identical.

Very often, however, the ultrasound results can’t determine whether your twins are identical or not, and you’ll need to wait until after the delivery. At that time, tests that compare the DNA of each baby will provide your answer.

Laura E. Stachel M.D. Obstetrician & Gynecologist