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.
Current as of: May 29, 2019
Author: Healthwise Staff Sarah A. Marshall, MD - Family Medicine Adam Husney, MD - Family Medicine Kathleen Romito, MD - Family Medicine William M. Gilbert, MD - Maternal and Fetal Medicine
Medical Review:Sarah A. Marshall, MD - Family Medicine & Adam Husney, MD - Family Medicine & Kathleen Romito, MD - Family Medicine & William M. Gilbert, MD - Maternal and Fetal Medicine