Personally I believe early in pregnancy - particularly at the morula stage - when the embryo is still a mass of unorganized cells - that abortion does not constitute murder. It's not much different than removing a mole at that point.
I understand what you're arguing at one level, but moles don't grow into a separate human being. That's pretty much the dividing line to me: there's simply not a process that moles, cancer, etc will become intelligent, and so we have to treat an embryo or a fetus like we would a whole human. We don't tend to argue that dolphins that are young enough don't qualify as dolphins, so we can kill them as we please.
I mean, as far as being able to survive and general intelligence, how old does a human tend to have to be to make it alone? At least five or six years? Humans are pretty insanely defenseless as very young children, even in a very benign environment. It seems pretty clear we're conveniently twisting our standards to overcome the moral objects of ending a human's life, yet we don't spend remotely the same effort to kill a more intelligent animal--pick just about any non-primate at one year old.
So, yea, if we as a society recognize we'd rather have abortions than have more unwanted/abused/neglected children, then that's really a pragmatic compromise we're making. If we're argue it's just a matter of instantaneous biology on intelligence or viability of life without support, we need to make some actual standard and apply it across the board. Maybe that means a one year old pig has more right to live than a one year old human. Then again, I don't think humans are any more special than humans make themselves to be, at the behest of defense from other humans most often.