Neph: One of my friends is atheist and pro-life. But that's a rare bird, for all that it arguably shouldn't be.
I'm pro-choice, and strongly in favor of the "viability" cutoff standard the Supreme Court originally used for when abortion becomes impermissable (for all that it's being undercut right now with claimed "fetuses can feel pain so let's roll the date back some weeks." The argument I prefer to use is a rather libertarian one. First, let's assume that life begins at conception, and that 2-day old fetuses have full US citizenship rights. This is extreme, but it's the safest concession to make. However, a woman "owns" her own body. Why can't she kick out a guest she doesn't want anymore? Yes, before viability, it will surely cause the fetus's death, but that isn't the mother's problem. This is the world being cruel. It's similar to the whole doctrine of "double effect" in end-of-life care where it's okay to pursue a treatment course that will almost surely lead to death if done for a good reason that is not related to that death - for instance, large amounts of anesthetic to alleviate pain (and so that someone can die peacefully), rather than dying slowly and in pain. The goal isn't to kill the fetus or the sick person in either case, it's a (terrible) side-effect. A similar example might be, say, your sick 80 year old mother coming back to her children and asking for lodging, food, and medical care. While arguably cruel, there is no "responsibility" to say yes. You can say "no, you're on your own, go talk with the government, you aren't my problem," even if this will likely lead to a faster death for mom. I think pro-life arguments usually use something like getting pregnant was a contract or agreement.... but even if so, these agreements can be revoked. If I invite people to my house to a party, I can kick them out afterward for any reason, because it's my house. Same thing with someone's body.
That said, I can certainly understand the fact that "it's a life!," if you believe this to be true of fetuses, to be a real consideration.