I am not sure, I distinctly remember a heated discussion I had a long time ago with a Catholic priest on exactly that point, you could "let" the fetus die, if it would save the mother (naturally) but you could not take any active intervention to "kill" (the word he used) the fetus. Only if it is absolutely clear that the fetus will never reach full term (like ectopic pregnancy, I am not sure about the terminology, but this is a pregnancy outside the uterus) is one allowed (under that doctrine) to intervene and "Kill" the fetus.
That deviation, of course open the door to "killing" the unborn (a terminology used by "lifer" who regularly use Syl's language of "butchers and murderers" when discussing physicians that provide safe abortion services). As far as Judaism is concerned, no life exists until the fetus has taken its first independent breath. Aquinas suggested that no life exist until thew end of the first trimester. I am sure that somewhere between these two dates, there might be a cross over date everyone can agree on.
Zeev
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