The organ donation comparison doesn't hold up. Someone needing a kidney is a stranger who arrived at your door through no relationship with you. Pregnancy isn't that. The child is in that position because of the relationship that created them, and the law already recognizes that parents owe their children basic bodily support. A parent can't stop feeding a newborn because it costs her sleep, effort, and her own body. We've never treated parental duty as optional.
And on personhood: if we genuinely can't say when a life begins, the safer legal default is to protect it, not permit ending it. Uncertainty cuts toward protecting life, not away from it. The unborn's status being hard to pin down isn't a blank check.
So the question remains what I said before. Not whether she controls her body, but whether that control includes stopping a separate, developing life. I don't think it does.