Oh the joys of an abortion debate.
I suppose when you get right down to it, it's a debate of whose rights override whose (that doesn't sound right, there needs to be a way to separate those "whose"... fuck English). So then you get into the problem of what rights anything has, and whether or not a fetus/whatever counts as a "person" or "human". Now "human" is defined as a living member of the species "homo sapiens", so that doesn't really help much. Technically, a single human egg is a human. So just throw that out. "Human" doesn't matter. Also, a fetus is a separate (technically parasitic) animal from its mother...
So you're left with "person". Some say it's defined by sentience (self-awareness; demonstrated in non-human animals on Earth) or sapience (mostly defined by abstract thinking and/or philosophy or whatever). Going by sapience, you'd have to somehow find out when a fetus is sapient... yeah, not helpful at all. Going by sentience, you're just cutting out some of very unlucky members of the human species. Which doesn't really help either.
Continuing on, that leaves one thing: animals of all kinds have rights. Obviously, this leads to problems like vegetarianism... And that's ignoring how you would separate animals from plants and such (for myself, animism solves it). That doesn't help matters much at all, and that basically leaves you with... nothing, actually.
So really... there's no clear-cut answer until we can solve how old a fetus has to be to be sentient. Anybody up for administering mirror tests to fetuses?