I'm pro-choice (I believe in souls, but I also believe that a soul in an aborted body would simply incarnate in another non-aborted body). But let me take a stab at an atheistic pro-life argument.
Suppose, in the future, someone will come up with an incontrovertable proof that abortion, at any stage in the life of the fetus, is murder (excepting when the life of the mother is at stake). If we continue on our pro-choice ways, and such a proof is offered, then we will have allowed millions of people to be murdered. However, if we outlaw abortion now there will be a certain amount of death and suffering (mainly due to illegal abortions), but nothing like the millions of murders that go on every year.
Without knowing whether such a proof is possible or not, isn't the safest course of action to assume a pro-life position? If a proof that the pro-life position is wrong appears, the worst that happens is some deaths from illegal abortion, and emotional suffering of women being forced to carry nearly all pregancies to term. If a proof that the pro-life position is right appears, however, millions will have been murdered. In the absence of a clear position, shouldn't we strive to minimize the worst-case scenario?
The argument turns on the probability (possibility) of a definitive "proof" being offered in the abortion debate one way or another, but it's not impossible. Slavery was believed by millions to be morally justified. There wasn't a single argument that reduced it to the morally reprehensible position it is today, but more of a gestalt shift in societal values. But that same shift may also happen in abortion- in 100 years, we may look back in horror at all the murders that took place. Our moral positions have evolved on other issues- forcing people to fight to the death, rules of warfare, interracial marriage, suffrage for women, etc. Why not abortion?
Wthether by proof or gradual evolution of values, might abortion be one of those things we look back at with horror, and if the possibility that we might look back at that way exists, should we assume a pro-life position, just to be on the safe side?