I'll throw my hat in the ring. Don't care to debate if fetuses are human or not though, because that's irrelevant to me being pro choice. I'm also spiritually inclined (Christian leaning) so not going to debate on the basis of being an immoral "atheist", as some here like to believe prochoicers are.
If its survival depends on another body, its survival or death should be up to the host donating those resources. That would mean limits to abortion when survival is possible outside moms body. It's not murder to not donate my organs, my blood, or any other body part, to ensure the life of another... period.
I see it as a private medical decision, because the woman donating her body can pay with disabilities or death or time or living standards. There's a lot of anti choicers that are hypocritical too, by not giving a shit about IVF legalization, while yapping about abortion.
I always wonder too, what prolifers are thinking when it comes to enforcement when it's concerning a "life" dependent of feeding off another.
If we were to agree it's a separate being, are mother's responsible for it like any other child? If she eats wrong, or not at all, is that child abuse? How about risky sex, with possible diseases? Is smoking and drinking then child abuse? What about taking over the counter meds (ibprofen) that can cause miscarriages? Foods like tuna fish? Dangerous activities? This all too quickly spirals into human rights abuses because person or not, it can not be a separate being from mom. Therefore, subjecting it to the same moral guidelines as other human rights, as if it's le any other individual, does not work out.
Finally, at least prochoicers are limited by common sense, unlike their opposition in many places im the world. Usually anti choicers make it too extreme because once again, it's tricky to give "equal" rights to a being that can't survive outside another human beings body. That's how we end up letting women die from ectopic pregnancies (where death is inevitable, but it's technically an abortion to help them), prosecuting women as murderers for their own suicide attempts, violating basic rights with undisclosed blood tests to convict them for substances, treating miscarriages with suspicion, and making raped girls risk death in child birth. Don't think it'll happen because that's too extreme? All these things have already occurred in the US in recent times. Look up Ohio on ectopic pregnancy. Look up child abuse accusations for eating a poppy seed bagel in PA.
So until artificial wombs can take over, not forcing women to donate their body to support another, that's where I stand.