Saving a Life or Stopping a Pursuer? (From Torah Tidbits)

Saving a Life or Stopping a Pursuer?  Last week we saw the Mishna that states that before the child is born we can abort him in order to save the mother's life, but once he is born or even the majority of the body is out of the mother then we cannot touch him since we do not push off one soul before another. This suggests that the unborn fetus is not considered alive and only receives the soul at birth. It is clear from the Mishna that the reason that before birth we do all that we can to save the mother's life but after birth we cannot interfere is that before birth he has no soul and only receives that afterwards. 
However, the Rambam brings this Mishna in his discussion of the laws of the rodef, the pursuer. It is a mitzva to stop the pursuer and not to be overly merciful. "Therefore" the Rambam writes (Hilchot Rotze'ach 1:9) "the Rabbis taught that when a woman is in danger during childbirth it is permitted to cut up the fetus either by hand or by a drug since he is like a pursuer to kill her. But if his head came out then we cannot touch him since we do not push off one soul for another and this is the nature of the world." The Rambam adds to the issue another element, namely that of the pursuer, and it is for this reason that we are allowed to kill the fetus. This does not appear in the Mishna and this addition changes both the understanding of the Mishna and the dynamic of the question. Since it now appears that were the fetus not a pursuer it would be forbidden to touch him and thus the Rambam appears to be of the opinion that the fetus is considered alive and only when he is a pursuer can he be killed. But according to this, the child is a pursuer for the entire time that he endangers the mother. Why then does the Rambam permit killing him prior to his birth but forbid it when the birth has reached a critical point, namely that of the head emerging? If the child is a rodef before the birth since he could kill the mother, then he should remain so even after until any danger has passed. These questions are not only academic but hold the key to understanding the halachic status of the unborn child.