I've been reading rather a large number of debates recently, in which a great many points were made, and it seems to me that the question of whether a fetus, quickened or not, is or is not a human being or ensouled is largely peripheral to the actual matter under discussion.
Suppose, say, that a Person In A Suit comes up to Jane and says "If you do not become pregnant and give birth, a meteorite will fall on Ann, here, whom you don't know, and kill her." (Assume for the purposes of this discussion that this makes sense, and that either Jane gives birth to a living baby or the meteorite falls on Ann, no wiggle room). Jane may or may not be married, may or may not have the resources to take care of herself during pregnancy (and/or the eventual baby), whatever.
This conundrum tends to get three sets of responses:
1. Jane should run out, get pregnant, and do her best to carry to term. Granted, it will be inconvenient, and she may or may not want to pregnant at this stage in her life, but if she doesn't, Ann will die. Dead is dead for a very long time. Even if Jane should miscarry, and the meteorite go ahead and fall on Ann, at least she will have tried.
2. Jane has never met Ann in her life. She didn't ask for Ann's life to be dependent on whether or not she gets pregnant -- for that matter, on whether or not she has SEX -- and this is a violation of her self on a fundamental level. If someone were to claim that "if Jane does not allow access to her body for pregnancy, she will be killing Ann," it would be analoguous to Richard Roe tying John Doe to a bomb and claiming that unless Jane gives him a lot of money -- or, perhaps a better metaphor, allows him to have sex with her -- before the time runs out, John will be blown up and it will be Jane's fault. (Which, no; it would clearly be Richard's fault for being a psychotic dick.) It is a fallacy, and Jane shouldn't endure something that is uncomfortable at best and life-threatening at worst simply because it may save Ann's life (note that if she miscarries or has a stillbirth, Ann dies anyway), or at any rate, shouldn't be FORCED to.
3. The responder may or may not know what they would do if they were Jane, but as they're not, they rather think Jane should make up her own mind as to what to do.
Now, at some point in the future, somebody may invent the uterine replicator, and we can trade our issues in for a new and different set of issues (probably centering around "should we use government funds to keep maintaining replicators containing unwanted embryos/fetuses?"), but until then, it seems that the above is what it would work out to even if people were to unilaterally say "yes, human life begins at conception."
(Which I don't, but as mentioned above, that's not particularly germane to why I'd still pick choice number three.)