To me personhood question is ultimately irrelevant because of body autonomy. No human has the right to latch on to a uterus and suck up nutrients without consent.
Nobody asked the fetus to begin to exist in the body of a person, who, at least in most cases, chose to have sex and knew or should have known about the risks of pregnancy. In most cases, excluding rape, the mother has significant responsibility for the person being there in the first place.
And even apart from that, "body autonomy" does not automatically trump the right of a person, assuming that personhood is stablished, to live. For instance, in most countries you are legally bound to help somebody who is in a medical emergency, even if that person does not have a general right to use your time and resources.