Abortion is typically - and incorrectly - presented as an issue of women's rights. It's not.
It's an issue of -human- rights - and not the mother's, in a typical scenario.
The question of whether or not abortion should be legal is, in fact, the question of whether or not a fetus is a human, and entitled to the legal protections of a human being. That's it. Not whether or not women should be forced to be incubators - we've already answered the question in court of whether or not we can be forced to act in a child's best interest against our will, as failing to protect a newborn infant from the elements is wrong.
Just one question. "Is this fetus entitled to the legal protections of a human being?"
Does the fetus have human value?
And the answer to this question is purely philosophic. Science has already answered the most obvious question - yes, a human fetus is human.
Does it have human value? Is it, for legal purposes, human?
What basis do you use to decide that?
Whether or not its capable of feeling pain? Well, that's not a fantastic definition of human, people exist who are not capable of feeling pain, and they're fully capable of objecting to a new non-human status.
Whether or not it has a fully developed brain? What constitutes "fully developed"?
Let's avoid the heartbeat, please, it's retarded.
Whatever measurement you decide to use, it's arbitrary. More, I suspect said measurement is going to be a rationalization for the conclusion you already decided you want to come to. More, someone else can pick an entirely different, and equally valid, measurement - after all, the argument is fundamentally philosophic.
This problem is fundamentally irresolvable in non-Objectivist philosophic framework.
An Objectivist philosophic framework DOES have a solution, albeit an ugly one: Because within the Objectivist philosophic framework you cannot have moral obligations generated by other moral entities, even if a fetus qualifies as a moral entity (that is, is human), you are not obligated to sustain it. Therefore you are well within your rights to remove it from your body. (But not necessarily to kill it. Allowing it to die? That is fine. Killing it? Back in ambiguous territory.)
One of many reasons I'm an Objectivist. Non-arbitrary and non-contradictory moral values.
(On an amused note, any in this class of solutions - that is, removing moral obligation - when consistently applied also removes any obligation on the part of the mother to the sanction of the father. It also removes the obligation on the father to both the child and the mother. All relationships are necessarily purely voluntary.)
Edit: Note my fundamental disagreement with Ayn Rand on this - in large part because she ignored the substantive problem of abortion, the definition of human life. While she claimed her yardstick was objective - rationality - her solution was to call a baby human at birth, which, while a convenient arbitrary point, is still quite arbitrary, and birth has no relation to rationality.