On the other side of that argument, a fetus does not have the higher brain function or consciousness that would allow it to experience pain. When an adult is put under general anesthesia for surgery we do not generally consider them to be "experiencing pain" even though the body is still reacting the damage as though they were conscious. They still have brain function, they temporarily lack the higher brain function required for the meaningful experience of pain. A very similar argument could be applied to a fetus.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.