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
Fraternal twins develop when two eggs are fertilized by two separate
sperms. The fetuses have separate placentas and amniotic sacs.
How this information was developed to help you make better health decisions.