Thanks for your question. The fact that twins are identical or fraternal is not determined by whether or not they share the same amniotic sac. Identical twins occur when one egg divides into two, resulting in the babies have the same, identical genetic makeup. Fraternal twins occur when two separate eggs are fertilized.
In utero, 90% of “identical” twins actually have two separate amniotic sacs. It is extremely rare and quite dangerous for identical twins to share one sac. There is a high risk of the two babies’ umbilical cords entangling at some point during the pregnancy.
Your girls could indeed be identical twins, especially if they look a great deal alike. That they share the same blood type isn’t an indication either, as it is quite common for siblings within any family to have the same blood type. The only way to know for certain is to have them genetically tested. Your pediatrician will be able to help you learn how to do that and the costs involved.
Back to Ask Tori »
Ask Tori RN®, by registered nurse and resident author of The Joy of Pregnancy, is a helpful and reassuring resource for parents-to-be.
© Copyright 2011-2012 The Harvard Common Press. All Rights Reserved. | Press Inquiry