Sanhedrin 72B says a fetus becomes a person when its head emerges.  But I have heard that abortion is justified based on the permission to kill a rodef (deadly pursurer).  But it seems to me that, logically, a fetus cannot be a rofef since it is part of the mother before it emerges.  I am looking for someone to clarify the status of a fetus before it emerges and clarify the concept of rodef as it pertains to abortion.