I’m pro-choice. But here’s an argument against the moral permissibility of abortion I haven’t seen:

Recall Singer’s thought experiment about the drowning toddler.

DROWNING TODDLER:
You are walking alone one day when you see a toddler drowning in the pool. You can save the child at no risk, but at significant personal expense. (Let’s say you’d have to ruin your $25000 suit.) Must you save the child?

Most people think the answer is yes, you’re obligated to save the child. That’s not to say that you lack the right to walk away, but that it’s wrong to walk way, even if it’s within your rights. So perhaps no one may force you to save the child, but nevertheless, you must do so.

What if we revise the thought experiment?

DYING BABY
You’re walking alone one day, when you see a baby about to die. The baby will starve to death or be asphyxiated unless you provide it food and air. It turns out you can do that at significant personal expense by carrying the baby inside your body for a few months, after which time someone else will be willing to care for it. Must you rescue it?

This is just  a sketch of an argument, so I won’t spell it out at great length. But it seems like if you think you’re obligated to rescue the toddler in Drowning Toddler, you should think you’re obligated to rescue the baby in Dying Baby. The personal cost of rescue is higher, as written, in Dying Baby, but we could tweak Drowning Toddler to make the cases more similar. At any rate, even if there’s a difference, it’s not obvious the personal cost is high enough in Dying Baby to excuse letting the baby die. After all, most people who like the Drowning Toddler thought experiment think it works even when iterated over and over again. Singer thinks you should give all your extra money to help others.

Now, one big difference is that the toddler in Drowning Toddler is indeed a toddler, while it’s not clear that a fetus counts as a baby. Still, as Singer points out, it seems arbitrary that we treat newborns as having rights but don’t treat third trimester fetuses as holding rights. The latter are less developed and still reside the womb, but they could survive outside the womb. Indeed, as Singer also points out, it’s not clear why we treat toddlers as having rights, given that that they are less like Kantian agents than many animals that we eat. Sure, toddlers have potential to develop into Kantian agents, but, then, so do newly implanted embryos.

I’ll leave that here rather than go through this stuff rigorously. Perhaps I’ll turn this into a paper someday.