the bottom line is that, legally, none of that actually matters. what matters is bodily autonomy.
scientifically speaking, we understand today that life is dna, which means that the church was actually right - life should be defined at what they call conception (and what i'd call fertilization), as that's when a new combination of dna forms. that's the science, here, and it's where any debate needs to start from.
and, i find it very difficult to argue that killing something isn't immoral, except in a situation where it's necessary to prevent imminent death, which isn't true for almost all abortions.
so, if you want to argue that abortion is immoral, i won't offer much push back on it. rather, i'm going to look you in the eye and tell you that i don't give a fuck.
the moral question just simply isn't absolute, it's just one consideration, and usually the one at the very bottom of the list, after you've sorted through everything else. and, when a woman decides to make that choice, she has a lot of things to sort through, the moral questions attached to the issue generally being of minimal, if any, importance. she needs to worry about if she can afford it, she needs to ask questions about the sperm donor (and if they choose to be more than that) and she needs to ask the question of if this is how she wants to spend the rest of her life, as the child will probably outlive her if it's born. all of these questions are of far greater importance to the individual's autonomy than the morality of the issue.
and, i really think that the left needs to stop making the pseudo-scientific arguments that it makes and instead approach the issue more realistically as one where morality is of marginal importance in the decision making process, because if they keep pushing these pseudo-scientific arguments, they're going to eventually lose them.