[Sanhedrin 72B][1] 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][2] is relevant but does not really grapple with the question above: 

Update2: We do not postpone the death penalty of a pregnant murderer. She is put to death and her fetus with her.


  [1]: https://www.sefaria.org/Sanhedrin.72b.14?lang=bi&with=all&lang2=en
  [2]: http://www.jlaw.com/Articles/preemb.html