The pro-life stance is based on emotion, not logic.
An tubal pregnancy is a common real-world example of what you described. This occurs when an egg is fertilized while still in the fallopian tube. It prevents viability in all cases for the zygote (or "baby" if you're pro-life), and is fatal for the mother if not treated. Tubal pregnancies account for about 2% of all pregnancies.
Treatment for a tubal pregnancy always involves abortion. If caught early enough, this can be induced by administering abortive drugs; otherwise, a surgical procedure is necessary.
If abortion is "murder," it's murder. It's not murder unless the mother would experience health complications, and it's not murder only if the fetus would develop normally. If egg+sperm=baby, and baby=person, then abortion is murder all the time.
This paints pro-lifers into a bit of a corner. Either pro-lifers are fine with murdering babies sometimes, or 2% of their women should be dying with EACH pregnancy. The latter certainly isn't happening, and you won't find any pro-lifers that will admit to the former.
There's only one conclusion, of course: pro-lifers don't really believe that abortion is murder. Their stance isn't one of morality then, it's one of control. By limiting a woman's reproductive choices, they hope to influence her sexual behavior. This is amply evidenced by the fact that pro-lifers are almost universally religious (hence the concern about sexual behavior), and overwhelmingly support abstinence-only sex ed, despite significant evidence that comprehensive sex ed results in fewer unwanted pregnancies--and abortions.