The remaining difficulty is what is the difference between a fetus and a "Yotzo Rosho" that the first is a Rodef and the second is not. We will go through two explanations here.
We will start with the explanation of R’ Chaim (on the Rambam). Briefly stated every Rodef involves two dinim – 1. to save the Nirdaf  2. to punish the Rodef –his life is forfeit.
R’ Chaim first proves that a fetus has the status of a Nefesh albeit not the same as a full human. Therefore the Rambam needed the explanation of Rodef for otherwise one cannot kill a “Nefesh”. This involves the first “din”. Once the fetus sticks out its head it reaches the status of a full Nefesh and the difficulty of “Ain Dochin Nefesh M’Pnei Nefesh” comes into play. An ordinary Rodef forfeits his right to life as a punishment for his intent to kill (the second “din”) but in the case of a baby where there is no intention to kill the second din does not come into play leaving us with the obstacle of “Ain Dochin…”.
The Seridei Eish is unhappy with this explanation. He admits that from a logical perspective of view R’ Chaim’s explanation is excellent but he feels that this cannot be the correct explanation of the “historical” Rambam. This is for the simple reason that we have no source for this “din” that the Rodef forfeits his life.
In an ultimate twist of irony, the explanation that the Seridei Eish himself proposes was already suggested (in part) by – R’ Chaim (this time in Grach Al HaShas also quoted in Even HaAzel). He cites the Rambam in Chovel U’ Mazik 8:16:
ספינה שחישבה להישבר מכובד המשאוי, ועמד אחד מהן והקל ממשאה, והשליך בים--פטור: שהמשא שבה כמו רודף אחריהם להורגם, ומצוה רבה עשה שהשליך והושיעם.
In this case the Rambam is using the term Rodef based on the Talmud Bava Kama which uses the same expression. The Rambam rules that even in a case of Pikuach Nefesh one still has to pay for any losses involved. The heter of Rodef frees one from having to pay for the donkey. The SE suggests that the Rambam is using Rodef solely to remove the obligation to pay the husband for the fetus.
This explanation is challenged by R’ Yaakov Kaminetsky ( Emes L’ Yaakov Nashim Nezikin). First, the Rambam is not talking about the payment of דמי ולדות. The Rambam in Hilchos Rotzeach is focusing on the problem of murder, not payment. According to the SE, the term מותר is problematic. If we were discussing monetary problems then the correct term should be “פטור”. Finally, the Rambam’s לפיכך is difficult. This implies a direct connection with the earlier Halacha. Not with a Halacha in farraway Chovel U’ Mazik. For this reason (and the fact that the SE doesn’t recognize the Rambam’s source as per the previous post) the SE’s explanation is difficult.
 See Prof. Steinberg's Encyclopedia of Medical Halacha (Heb.) S.V. Ubbar, and Hapalah for a complete discussion of all aspects of this problem.
 According to R' Chaim this is different then the regular heter of Pikuach Nefesh for it applies to a non-Jew as well for whom the law of Pikuach Nefesh does not apply. The Chazon Ish rejects this for the Gemara in Yoma (85b) tries to use Rodef as a source for Pikuach Nefesh as a whole proving that Rodef is in fact similar to the typical Pikuach Nefesh. I do not understand the Chazon Ish's difficulty. R' Chaim will simply answer that every Rodef contains within in it two-dinim 1- to remove the problem of "Ain Dochin Nefesh.." and this is different then every other Pikuach Nefesh. 2- that we overide the prohibition against murder and it is from this second din that the Gemara in Yoma attempts to learn out to all types of Pikuach Nefesh.