were Shifra and Pu'ah required to be moseir nefesh to save unborn babies?

R’ Noson Gestetner in his sefer on chumash asks what right the meyaldos ha’ivriyos, Shifra and Pu'ah, had to be moseir nefesh to save the Jewish babies that were not yet born (assuming, like the Rambam paskens, that one is not allowed to be moseir nefesh if one is not required to do so).  The reason why murder is a yei’hareg v’al ya’avor is because of the sevara of mai chazis d’dama didach sumak tfei – saving one’s own life cannot come at the expense of someone else’s, as no one life is more valuable than another.  However, the Mishna in Ohalos (ch 7) writes that a fetus may be killed to save the mother’s life.  We see from the Mishna that a regular person’s life is in fact more valuable than that of a fetus.  So why were the meyaldos allowed to sacrifice themselves for the sake of saving the not yet born babies?

The question assumes, as Rashi writes (Sanhedrin 74), that a fetus is not a “nefesh” relative to a regular person.  However, the Rambam (Rotzei’ach 1:9) formulates the din a little differently:

אף זו מצות לא תעשה שלא לחוס על נפש הרודף. לפיכך הורו חכמים שהעוברה שהיא מקשה לילד מותר לחתוך העובר במיעיה. בין בסם בין ביד מפני שהוא כרודף אחריה להורגה. ואם משהוציא ראשו אין נוגעין בו שאין דוחין נפש מפני נפש וזהו טבעו של עולם:

R’ Akiva Eiger on the Mishna already points out that according to the Rambam, the fetus can be killed  only because it has the din of a rodef, not because it is not a “nefesh.”  R’ Chaim al haRambam works out how the Rambam fits with the gemara, but be that as it may, according to this approach I’m not sure R’ Gestetner’s question gets off the ground.  The rodef here was Pharaoh, not the baby, as if not for the special circumstances of Pharaoh's decree, there would have been no danger.