There's also an argument that it doesn't matter. An unborn child is 100% feeding off of the parent carrying them. Nobody has the right to force that choice on anyone.

If you're going to say anything other than unconditionally legal, you need some really clear legal definitions on something, but you certainly can. Like you could define viability as if you delivered it on the spot, you'd have a fully-formed baby with lungs that are ready to breathe, and otherwise unlikely to need life support. You could define the first 6 months of pregnancy as inviable.

You could define the burden of proof in a way that protects doctors, maybe someone trying to already wrongdoing needs to prove that no reasonable physician would agree with their judgement. You could even limit who has standing to take legal action, because some random person on the street isn't party to it at all.

I'm not saying that "if the doctor and pregnant person agree, it's legal" is bad, but there are certainly other reasonable options, that I think would play out similarly in practice. Like I'm assuming a doctor about to deliver a baby wouldn't likely entertain a request for an abortion instead, nor would they likely get one.