Abortion is always justified by one of two reasons:
#1: That sapience determines moral value. This makes fetuses, being nonsapient, morally equivalent to a rock or other inanimate object. They're just some flesh, after all.
The problem here is that even if sapience were the basis for value, it doesn't follow that a nonsapient object would be valueless. Fetuses have the ability to develop sapience, an inherent capacity which is part of their nature. They may not be able to immediately exercise sapience, but this also applies to someone who is under general anesthesia, or in a coma. It clearly isn't acceptable to kill them!
Even if we accept that only active sapience counts, it's not clear that this applies to certain other things which we value. It is considered acceptable to kill animals, but it is not considered acceptable to kill children below two years of age, although their mental state isn't advanced much beyond than a chimp's. To assert otherwise would be considered abhorrent.
Finally, even fully-grown humans have varying degrees of sapience. The lives of those with Down Syndrome or other mental disabilities would therefore not be as valuable as those of healthy people.
#2: That women have an absolute right to complete control over their bodies. Therefore, since the fetus resides within her body, she has control over it, and may kill it if she desires.
This simply ignores the rights of the fetus. Yes, women are generally accepted to have rights over their bodies, but fetuses, being young human beings, also have a right to life. A woman's right to her body doesn't trump a human's right to their life. This is demonstrated by supposing that the atmosphere has developed sapience, and decides to suffocate all humans within itself, though they pose no threat to it. I think it is pretty clear that the atmosphere is not acting morally here.
A final point for those who think the right to bodily integrity is stronger than the right to life. Yes, women do, under your logic, have the right to kill their unborn child, and no one has the right to stop them, but this doesn't mean it is any less monstrous. You aren't doing it because the fetus poses a threat or hindrance to your body, you're doing it because you don't want the fetus to grow up and for you to be forced into an obligation. It's the same as the government rounding up and shooting homeless people on the grounds that productive citizens will not longer have to support them.
There. Can we all agree that abortion is wrong now, and get on with our lives?