Well, it's not the simple debate either side claims. To clarify, two questions:
1) Under what circumstances and conditions is it justifiable to take a human life?
2) When does human life begin?
Traditionally, inconvenience/poor timing/negligence and "I don't wanna" have not been acceptable standards for killing someone. Nasty little names like murder, manslaughter, and negligent homicide have all been applied to such merry activities.
On the other hand, every legal system in the country has in it some provision in it for "he/she needed killing", typically wrapped in a self-defense statute, and usually with phrases like "threat to life", "engaged in a felonious act", or "reasonable threat to physical safety of self or others".
The felonious and assaultive abilities of an infant still in the womb are fairly minimal. There certainly exist medical circumstances where some variation of the dread choice "kid v. mom" needs to be made, as one is merely choosing coffin size, not the impending presence of coffins; and similarly, there exist at all points in life "quality of life" issues for a patient that we as a society have come to realize justify "plug-pulling".
Then we come to the challenging question - "At what point does human life begin?"
Surely, someplace between mitosis and age 70 we can come to an agreement that a human life, mostly worth protecting, is in progress.
The medieval definition could be considered "at birth" (whether natural or by primitive c-section). But the middle ages were a *long time* back, and survivability has advanced a little bit since then.
I think all the sane among us can agree that "once born, is life" and we can move on from that point with a narrower question - "Where, between mitosis and birth, does life begin?"
I would suggest that a reasonable expectation that if removed from the womb unmutilated by the process of infant survival indicates that the fetus/infant is a pretty clear indication that fetus/infant has achieved personhood. And killing off people without a damned good reason is murder, morally.
Being female, historically, has until quite recently not been a get out of jail free card. And in the presence of todays wide range of effective contraceptives and a 4-5 month pre-viability window (it ain't independently viable, thus not yet a person), there is no reason that it *should* be.
Now, I expect I'm in trouble now from both the "abortion is evil" and the "women are the only one that can make this decision, abortion is a RIGHT" crowds - and that's a good sign, because if you piss off the extremists at either end of a debate...you're probably making sense.