The abortion question comes down to one thing.... when does a human life begin? Everything else is irrelevant.
Life, in the biological sense, cannot begin at conception obviously, since a dead sperm or a dead ovum cannot make in living diploid human. Much less can it begin at birth for obvious reasons. Life doesn't begin it's passed on, so the question lacks coherence and we can easily reject a life-based analysis for resolving this question. That is, of course, unless you are using some specialised defintion of 'human life.' But this means this question resolves itself to how it is that we define the term.
Now we can sensibly ask when some form of legal personhood begins (which some people might mean by 'a human life'), which at common law is birth (but being law subject to re-definition). That this is arbitrary, and therefore philosophically unsatisfying, has at least the benefit of producing a definitive answer.
Philosophically we might consider the fact of individual human consciousness (which does have a beginning) that might more sensibly be considered in resolving the ethics of abortion. The question would be, at what point does the possibility of some form of rudimentary self-awareness begin? Which, I suspect, would lie at some point after conception, but before birth.
HOWEVER, there is also the question of the individual human consciousness and the bodily autonomy of the human who is to host the fetus. To sit around and insist that "the abortion question comes down to ... [the question of] when does a human life begin" or even "when does an individual human consciousness begin" is a characteristic of those whose bodies are unlikely ever to be called upon to act as a host. The abortion question, for many in the debate, comes down to one thing, whose has the right to decide what a woman may do with her own body (including carrying or not carrying a fetus from conception to birth). Everything else, they might insist, is irrelevant.