Maybe it's straightforward to discover when the fetus can feel pain, but it's not straightforward that being able to feel pain should be the cutoff point.

I always thought this was a pretty straightforward issue from a rationalist perspective - we have the technology to figure out whether the foetus has an active brain and can meaningfully perceive pain.

I see no reason to believe that you can in general kill a person because the person can't perceive pain. Congenital insensitivity to pain is a thing.

"Active brain" is also a term that wide open.