I know that with the election of a democratic president and democratic majorities in the House and Senate, abortion has become an almost passe issue in many quarters. However, this election caused me to think about the issues that make me such a consistently progressive voter and why I hold the beliefs I do. Women's right to choose is, to my mind, fundamental, and I have always gotten very angry when prolife advocates say that women "made their choice" when they decided to have sex. Then I realized that it wasn't just the blatant sexism or the idea of babies as punishment that bothered me. Even assuming personhood begins at conception (a view I do not share) the prolife argument is out of sync with history of the Western bioethics community and the discipline's approach to the idea of consent.
First, to explore the idea of consent a bit more in depth, imagine for a moment that you have cancer. You have undergone some chemo treatments, but you know that there will be many more. You decide you don't want them, that you'd simply rather take your chances with the cancer. Just because you started the treatment, doesn't mean you have to finish it. Part of the nature of consent is that it must be continuous; if you revoke it (assuming you are legally mentally capable), the treatment must be stopped as quickly as possible.
Second, if you assume that both the mother and the fetus are full and distinct persons, then there is a medically definable relationship between them; namely, the mother's body is serving as a life support system for the fetus. Now, in order not to give the fetus more rights than a fully developed homo sapien, let's compare this to a theoretical relationship. Imagine if the only way a woman could ensure that her 6-month old child would live would be to hook up a machine to her body that would cause all the symptoms of pregnancy, and remove nutrients from her blood. She would drop the blood off at the hospital every few weeks, and the child would live. In such a situation, the vast majority of women would hook themselves up in a heartbeat. Two points are important to note, however. First, for the government to get involved would be a radical departure from history. For instance, just because you'd make a good kidney donor doesn't mean the feds can take yours away from you and give it to someone else, even if it would save a life. Second, if at any point in the time period, for any reason, a woman were to decide to stop,the doctors would have to accept her decision. She would be revoking her consent. If you can revoke your consent to act as life support for your infant, you should also be able to do it for your fetus.
This is not a good argument for the right to choose beyond viability, for obvious reasons. This argument also raises the question of whether it wouldn't be more morally palatable to society if abortions simply involved inducing labor before viability, thus making absolutely clear the reality of the fact that there's no need to cut up fetuses at that point in order to abort them, because the whole point is that women's bodies aren't life support systems.