Answer:
just that they are conjoined, or joined together by a body part. if they are conjoined at the head, it could causes brain damage and depending on how much of a brain they share they will have to do surgery and during the surgery one of the babies will die. if it is not too severe, a quick separation won't do anything, but could causes mental problems during birth. i am not sure what happens by being conjoined at the heart, though. i think that is either impossible, or the baby(ies) is born dead.