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 pursuer). But it seems to me that, logically, a fetus cannot be a rodef 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 (e.g., is it a person? does it have free will?) and clarify the concept of rodef as it pertains to fetus and abortion.

Update: This article is relevant but does not really grapple with the question above: http://www.jlaw.com/Articles/preemb.html
Update2: We do not postpone the death penalty of a pregnant murderer. She is put to death and her fetus with her.