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Twin Pregnancy Types

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Identical twins come from a single egg that has been fertilized by one sperm. For unknown reasons, the fertilized egg splits into two embryos during the first stage of development. In the mother's womb (uterus), most identical twins share the same placenta. (They get oxygen and nutrients from the mother and get rid of wastes through the placenta.) But they usually grow within separate amniotic sacs. In rare cases, identical twins share one amniotic sac.

Fraternal twins develop when two eggs are fertilized by two separate sperms. The fetuses have separate placentas and amniotic sacs.

ByHealthwise Staff
Primary Medical ReviewerSarah Marshall, MD - Family Medicine
Specialist Medical ReviewerWilliam Gilbert, MD - Maternal and Fetal Medicine
Last RevisedJuly 8, 2011

WebMD Medical Reference from Healthwise

Last Updated: July 08, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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