I can try to address this. Not that my view, of course, speaks for everyone who is pro-choice, but I still believe it is possible to logically address that point. (in a really long winded way, granted)
I think the right of abortion doesn't dervice from the fetus not being a living being. I think it derives from a conflict of the rights of the fetus and the mother. When I speak of rights, they are in no sense absolute. Absolute rights would be simple. Most ethical conflicts come about because there are two sets of rights which are in conflict. In this case, the fetus has a right to life which is in conflict with the mother's right to determine the use of her own body.
Current law would therefore support that the mother's right to use of her body overrules the fetus's right to live. Confusion arises when people try to solve this conflict by stating that it is purely based on the strength or weakness of the fetus's right to live. That is, if the fetus is a 'human being' it has a 100% right to live. If the fetus is not a 'human being' it has a 0% right to live. I believe that is oversimplifying matters and creates the confusions that you referred to.
Imagine an analogous case. (I want to put in a disclaimer here. I've used this argument before and been accused of trivializing abortion. This is purely an analogy based on a similar conflict of rights. I'm not at all saying these are morally equivilent situations. ) Let's say you had a kidney disease that neccessitated trasnplant in order to prevent your death. For the sake of argument, transplant is the only treatment which will work. Unfortunately, you have a very rare blood type. So rare, in fact, that only one person could provide you with that transplant. Are they morally obligated to? You will die without the transplant. Your right to live is in conflict with their rights to the use of their own body. Most people, I think, would say that while giving you the kidney would be a good thing to do, we shouldn't legally require the person to do that thing. That is, we would say that that person has the absolute right to determine who gets use of their body.
Now, the abortion issue is complicated by the fact that the mother caused the conflict in the first place. She, except in the case of rape, was involved somehow in the conflict of rights occuring. That's another dimension to the argument, and while important, doesn't really bare on your original question. (and this is long enough without going there.)
So, anyone, that's the conflict of rights. That's why, I believe, someone else could be charged with killing the fetus. They have no right in conflict with the fetus. Even if, in relation to the mother's right to her body, the fetus's right to live is lesser, it is still no small thing in and of itself. And when that conflict is taken out of the picture, another person has no right to kill that fetus. After viability, the conflict is also gone. The fetus can be 'removed' from the mother without killing it. Once that can happen, there is no conflict in rights.
Based on this, personally, I'm in favor of the right of abortion until medical viability (currently 23 weeks gestation).
