The Fifty Minute Hour:

Even if we were to accept the premise that a fetus is a person, it may have a right to life, but it doesn't have the right to subject another person against her will to painful and dangerous medical consequences. If I needed a kidney transplant, I might be able to get the organ I needed to survive by stabbing someone and forcibly removing one of theirs. But even if they recovered with no ill health effects after the attack, I'd be guilty of a moral (and legal) wrong for subjecting them to it in the first place. Those people who want to consider fetuses person must remember that even another person with a right to life doesn't have the right to that life at the expense of another person, not even if that person is the child's parent. Being pro-life can't mean being in favor of saving one life by allowing it to hold another hostage for 9 months.

a very good point.