We could make it not opinion with ease. Make the test:

“Can the fetus survive without the host body?”

That’s a medical question that will slowly move toward not aborting ever. And it solves the medical issues as well. “This fetus is killing the host” always allows for removal, because we can either keep them alive, or it can’t survive.

Then the folks who want more babies to reach term can focus on improving medical technology instead of getting involved with the mess that is people’s love lives.