Anti-choice apologists have a tendency to focus on the question of at what stage a baby is a ‘person.’ Of course, for many, due to their religions positions, they will maintain that the baby is a person as soon as it has a soul, and this, they will say, happens at inception (an odd time, since many fertilized eggs fail to implant, and die without the woman ever knowing she was pregnant), or at some other specific point.
Atheists on the other hand might argue that it occurs when the brain begins development, or at some other measurable point.
However, I find the question to be quite irrelevant to the debate on abortion choice. Obviously if the baby is not a person, nobody objects to aborting it. Suppose on the other hand that the baby is a person. Then the question is: does one person have the right to live by being parasitically hooked up to another human being? Does that other human being have the right to have someone who is hooked up to them parasitically removed?
I would have to say that the answer to the latter question is yes. The irony here is that most people who claim to be pro-life would be mortified at the idea of the government requiring them to donate blood, or bone marrow, or anything of the sort. Yet, if you claim that the government can force a person to allow another person to remain physically connected to them for survival, you must, to be consistent, also maintain that any person can force another person to give them bodily materials for survival. Or, alternately, the anti-choice apologist could argue that the mother-baby relationship is a special case. However, I don’t buy it as a ‘special case.’
For me, it is one or the other: either everyone has the right to decide whether their body will be used to support the life of another, or nobody does.
EDIT: I’d like to add a quick explanation for why I referred to the foetus as ‘parasitically hooked up’ to the mother. Yes, pregnancy is essential for the continuance of the species (at least until we can figure out how to grow humans in labs). However, in principle, both the foetus and the parasite are foreign organisms which are deriving their nutrients and survival from the human. I wished to emphasize this similarity.