My two penneth worth: abortion is complicated because it confuses a few normally separate issues:
1.Should any human being be compelled to use their body to sustain the life of another?
2.How do we measure quality of life and who gets to make those decisions? (Essentially, when we give/withhold medical treatment, end of life care and euthanasia)
IMO debates over when life begins just obfuscate the issue. My uncle Fred is undeniably alive, but most people would agree that I should not be compelled to donate my bone marrow to him, even if I do appear to be his only viable match. Obviously it would be very nice if I did and, under some situations, it could be argued I have a moral imperative to do so - but this would be a matter of personal conscience and not law. Even if we up the stakes a bit and say Fred is my son rather than my uncle, it is still difficult to imagine a world in which I am legally compelled to donate to him.
So my personal opinion on (1) is that there is rarely ever a situation where one human should be legally compelled to use their body to support the life of another human. In theory this means I support ending a pregnancy at any time up until birth.
I think the logic for this is quite simple when taking about medical abortions because the harm to the baby is indirect: the woman's body simply stops providing any support and the baby is either no longer able to survive independently or, in the case of induction, potentially able to exist independently of the women anyway (with possible medical support). I suppose this is a bit like if I get matched to donate some bone marrow to Fred, but decide not to go through with it.
This nice clean logic gets muddied when I think about surgical abortions. IMO we are now into the difficult world of quality of life and euthanasia. I think doctors are probably best placed to evaluate this on a case by case basis. However, in all cases, where the there is a conflict between the life of the women and baby (pre-eclampsia for example), the host should take precedence.
TLDR: it's a bloody complicated issue, but we should approach it from the assumption that no human being should be compelled to use their body to sustain the life of another human being.