Multiple pregnancy usually happens when more than one egg is fertilized. It also can happen when one egg is fertilized and then splits into 2 or more embryos that grow into 2 or more babies. When one fertilized egg splits into 2, the babies are called identical twins. A multiple pregnancy is a pregnancy where you're carrying more than one baby at a time. If you're carrying two babies, they are called twins. Three babies that are carried during one pregnancy are called triplets. Early signs of a twin pregnancy include severe morning sickness, quick weight gain, and more breast tenderness. You may also notice an increased appetite or extreme fatigue. Plus, those who have a twin pregnancy may begin showing sooner.