Judith Jarvis Thomson's "A Defense of Abortion" is sometimes said to be the most-reprinted article in philosophy, and I believe it. It's one of the most influential papers in all of applied ethics, and several of the arguments Thomson makes have become standard moves in completely unrelated discussions.
One of Thomson's claims is that it would be morally indecent to have an abortion in the ninth month for fairly trivial concerns but that we shouldn't expect a young teenager in her first trimester, pregnant by means of rape, to go through with a pregnancy. She thinks it would be a wonderful moral decision to choose to go through with it, but it's more than we should expect. Philosophers regularly speak this way. They find a middle ground between what is wrong and what is morally required. That range includes anything that would be morally excellent to do but not morally required. This does fit with a lot of people's moral intuitions. There are sacrifices that would be morally admirable to make, but no one is really obligated to make them. This class of actions is called supererogatory. Thomson is saying that it's supererogatory to go through with a pregnancy in some conditions, but it's morally obligatory to go through with it in other cases.
What struck me as odd as I was reading the paper again this time around while preparing to teach it was her use of Jesus' parable of the Good Samaritan to express this view. She says the ninth-month abortion in the above paragraph wouldn't even be an example of a Minimally Decent Samaritan. We should expect more morally. But going through a pregnancy in the other case would be an example of a Very Good Samaritan, i.e. well beyond the call of duty. I'm not going to dispute the possibility of distinguishing between a range of cases, with some supererogatory and some morally obligatory. It does seem strange to use the Good Samaritan parable to do so, however, since Jesus' point in that parable is that you ought to love your neighbor as yourself, and your neighbor is anyone in need, which means you ought to go way beyond what you thought you were obligated to do, and this is even in cases involving complete strangers whose social position means you wouldn't normally even rub shoulders with the person. In other words, Jesus is at the very least minimizing the category of supererogatory actions. He doesn't explicitly deny that there are such actions, but it's hard to avoid the impression on reading the parable that he thinks most actions philosophers would classify as supererogatory as actually morally required.
That suggests an interesting response to Thomson's argument. What about those who don't hold to a view like Thomson's about supererogatory actions? What we ought to be as good as we can be? What if we ought to do as much good as we can do? Thomson's intent is to assume for the sake of argument that a fetus has full moral status and a right to life, arguing then that there are still reasons to think abortion is morally permissible under certain conditions (and as she goes it becomes clear that those conditions aren't just extreme ones like rape but include any case of failed contraception, provided that the abortion takes place early enough in the pregnancy). There are lots of places people might question her argument, but one place I hadn't thought about was to question her reliance on there being a wide range of supererogatory acts. If not, then you might concede all her other points and still oppose abortion. If you think it's morally better to go through with a pregnancy, as Thomson concedes (and many pro-choice people have since then), then once you deny supererogation you end up with a moral obligation not to have an abortion, and this has nothing to do with fetal rights (Thomson is no longer assuming for the sake of argument that the fetus is a person when she gets into the Good Samaritan stuff).
So Thomson's argument gets turned on its head. She started with an argument defending abortion even if the fetus is a person with full moral rights, and once you deny the supererogation premise you end up with an argument that abortion is often immoral, without assuming anything about the personhood of the fetus. It does involve a pretty controversial premise, but it's an interesting argument nonetheless, and there are lots of people who deny supererogation (or at least should do so given their other commitments).