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Twin Pregnancy Types

Picture of twin pregnancy types

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.

By Healthwise Staff
Primary Medical Reviewer Sarah Marshall, MD - Family Medicine
Specialist Medical Reviewer William Gilbert, MD - Maternal and Fetal Medicine
Last Revised July 8, 2011

Last Revised: July 8, 2011

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD - Family Medicine & William Gilbert, MD - Maternal and Fetal Medicine

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