In the link that huh? so kindly supplied, that was the case. My issues with that was that his Kasha on Reb Moshe is based on a his own brand new Pshat that he learned into the words of the Rambam. From the Gemara there is no basis for his explanation/Kasha. His own Heter is also a little puzzling. He wants to consider someone about to die as a Mechuyav Misa. How does that differ from someone that would without a doubt be killed by the enemy surrounding the village, where we aren’t allowed to give him up?
To add a little to the subject, the Gemara seems to say that the reason we would kill the fetus to save the mother is because it is not yet considered a full fledged person. The Rambam however, says that we kill it because it is Rodef. Rabbi Akiva Eigger asks this and says that although killing a fetus would not beget a Misas Beis Din, it is still a transgression of Retzicha. Therefore the Rambam says that we would kill it as a Rodef. Reb Moshe points out in another Teshuva that a Treifa has the same Halachic status as a fetus. Therefore in the case that was discussed then, the Baby A, who was definitely a Treifa, and a Rodef can be killed to spare the other one. Reb Moshe didn’t say this because, probably, in a different Teshuva, he answered the Rambam differently, that when they are co-existing side by side then neither is the Rodef. Whereas, however, it is still a fetus he is being the trouble maker. In the above link he seems to differentiate based on if it is depending on the other, and that makes it not be side by side.