> Let's start with the hypothetical that a pregnant woman is expected to die unless her fetus is aborted... Who would you choose to murder?
First, how ever cruel and wrong doing so may be, letting nature take its course isn't murder - it's something else altogether (sometimes morally defensible, sometimes not).
Second, your hypothetical situation is a well-covered case in situational ethics -- "whose life to save when not all can be saved". I know in this case many Catholic hospitals would save the baby and let the woman die -- so they can baptize the baby and save its soul (so they believe... correct me if I'm wrong). I disagree.
In my ethical framework, the woman's life is the one worth saving. Between the two of them, it will be more devastating/hurtful overall for her to die than the baby... not to mention devastation/hurt to those who love her. Therefore it is more loving to save her. And if the baby dies, I believe he/she goes to heaven (that's not justification for infanticide or abortion, just that based on scripture, I don't agree with the Catholic belief that the baby has to be delivered and baptized for him/her to receive salvation).
> It hinges on an arbitrary "It's alive!" moment that's not really debatable, so much as a gut feeling.
... except that science has pretty much established when a new organism begins for every other species. It's well known. People just don't want to accept the moral ramifications for human babies.
Consider someone on a ventilator who is otherwise a vegetable -- yes they're alive, and stabbing them would be murder. However, it's not necessarily murder to let nature take its course -- to let them die by unplugging the machine that's keeping them artificially alive.
I think it's your ethics (or perhaps your gut feeling, as you put it) that doesn't allow for a binary "alive or dead" state. But biology is pretty clear on when an organism is alive or not.
Yes there are edge cases when tissue lines are kept alive for years, or organ transplants are done... but no one disagrees about how alive or dead someone is in those cases. The point is whether a human fetus is an alive person - that's where the rubber hits the road ethically. And biologically there's certainly no question in that case. He/she is a human and he/she is alive -- that's the moral starting point. Anyone who says otherwise is equivocating to avoid the moral ramifications.