Standard for Personhood
(This is my first post in this community so my apologies if this topic has been discussed before. If it has please respond with a link to the post.)

Abortion proponents often make the claim that a fetus is not a person. The main reason is that the fetus lacks the ability to carry out some function that a recognized person does. The fetus can't survive on its own, communicate, reason, feel or exercise any rights they might have.

The problem with this functional argument is that the functional deficiencies of the baby in the womb also exist for the baby outside of the womb. A newborn cannot survive on its own, communicate (intelligently), it cannot reason (as we understand it) nor are its senses fully developed. Yet most pro-choice people would agree that a newborn has the same right to live as any other person. Not only that, not all "persons" have all of these functions (i.e. they are handicapped or infirm.) Yet we don't consider them as less of a person. Even perfectly healthy persons have moments where these functions aren't working (sleep, unconsciousness, temporary insanity, substance-induced stupors). But nobody considers them non-persons or less of a person during those moments.

My question for pro-choice people is this: what defines personhood? Is it function and ability? If so, why not provide less rights to the less abled and more rights to the more abled. If personhood is defined by something else, what is it?