If you twins share one sac - they are identical - no doubt as the only way this can happen is if the egg splits.    There are 2 types of twins that share the same sac (outer membrane).  Monochorionic - monoamniotic (MOMO) and mono chorionic diamniotic.  My idential twins were the latter.  What this means - MOMO twins share the same outer sac (the chorionic sac AND share the same amniotic sac.  this is a very very high risk pregnancy - they first thought my twins were...