How do self-described ancaps (who say they are anti-abortion) intend to enforce that principle as a matter of law?
…as asked by a libertarian historian on Facebook….

…but answered here:

How do we enforce the law against murder in a foreign country? We don’t. Someone else does.

The difficulty regarding abortion is that it is private, within the womb. This makes the issue become a micro-political jurisdictional problem. Most anti-abortion libertarians I talk to are not interested in a police state to track every pregnancy or period. That’s absurd and they know it. Only one lunkhead in a hundred suggests it seriously.

We all know that prosecution of murder usually depends upon a corpse. Most murders go unsolved and unprosecuted — that’s my take-away from 600,000 people going missing each year in the United States (some unknown number of whom are murdered) compared to the relatively minuscule official murder count and a falling rate of solutions to police-designated homicides. Abortionists in a pro-life society would no doubt go to great length to dispose of corpses. Right now, however, the corpses are incinerated, put in dumpsters, used in industry for cosmetics, medical experimentation and drug development, and much more — all legal. In a legal environment where that goes away because occasionally prosecuted in the courts — abortionists would likely become quite clever in disposing of bodies.

In the hypothesized anarcho-capitalist (ancap) society, certain crimes would be rarely prosecuted. Just as today. There’s always a selection bias in any system. My guess — and this is gleaned not merely from my own speculation also from talking to ancaps who are against abortion — is that they don’t expect it to be often enforced — just as Ron Paul, a minarchist, doesn’t expect laws against abortion to be enforced often against individuals. But ancap anti-abortionists as a matter of principle aren’t going to pretend that poisoning and grinding up fetuses in or out of the womb is anything but murdering a human being, because they see themselves as the opposite of callous nazi-like progressives, who sacrifice offspring for their own pleasure and convenience, and perhaps (this is something I’ve encountered in discussion, left and right) as a mass sacrifice to their pagan deities.

Much of the oomph of the question goes to the problem of who has standing in Ancapistan. Well, that question has been explored in the literature, but a lot would probably depend upon the form anarcho-capitalism takes. Writers as different as Stephan Kinsella (not anti-abortion) and David D. Friedman (I don’t remember his position) admit that “anarchist” societies could be quite diverse, legally. This ends us in Hoppe-land, actually, where private societies differ in complexion, and every society would have issues upon which expulsion from said society would be de rigueur. A controversial position, but hard to argue against on the basis of elementary libertarian principles.

I am not a professed and committed anarchist, so I regard these questions as interesting avenues to explore. I am “against abortion” somewhat like I am “for liberty,” as a general position. Specifics often get difficult. We should explore these questions rationally, if we can.

Cultural schisms in the libertarian movement make this difficult.