The entire argument here is that if we consider a fetus a person, then we should apply self-defense laws to pregnancies.

That is certainly one part of the issue here.

Dont get me wrong here, I do absolutely understand your viewpoint here I think. Especially as regards the slope that lawmakers can use to slip down. This is a tricky and nuanced subject, which is why I'm largely in favor of leaving it the fuck alone. That's kind of the context of the entire post that we're debating in the comments of. If a fetus becomes legally declared to be a distinct person then suddenly half our legal code can be used in absurd and self-inconsistent ways. Currently that is not the case but some people very much want it to be that way.

Personally, I say a person becomes a person when they prove themselves an independent thinking being and they retain that status until their death. Babies, generally speaking, become independent thinking beings upon birth. Before that they are still biologically attached to the mother, thus not independent and therefore subject to the will of her person, and after that they move and think on their own and have become their own being. A person who is in an unresponsive coma is still considered a person because they attained personhood and have not yet died, but even today there are legal loopholes for family to decline further care for the comatose person. That probably won't change. If your family has hope for you you'll stay alive and if they don't then they can order your death, I don't really see how you get around that in a world where comas still happen.

Right now we have a shaky, but stable enough legal framework around this sort of thing that's been put together over a couple hundred years of people thinking about this. But if we go poking at things that are core to the legal code, such as "what is a person", things start falling off of it.