“Personhood is not a separate, added quality that gives human beings significance.”
That argument is somewhat tautological. In this context “person” and “human being” are two terms for exactly the same thing. So what have you proven? The important question is when, exactly, does a human egg become a human being? Is it when it becomes “viable”? When it’s “inevitable”? When it’s capable of free will? When it looks like a person? When it looks too cute to harm?
According to US law, when most of the head of a developing human “something” is outside of its mother’s body, it suddenly, mystically, becomes a human being, endowed with full human rights. This arbitrary and quasi-religious definition is sanctified as “settled law”. That’s sure to cause bitter problems, and it does.
All of that is why this push to recognize that one becomes a human being at conception is most scientifically and logically sensible. Other reference points along the development curve are too arbitrary in nature to even BE “points” at all, and the “magic birth canal” argument currently codified in Law is simply ridiculous on its face. The notion that one transitions from “not a human being” to “a human being”; from “having no rights” to “having rights” by passing through a tube — utterly stupid.
On that score, I would suppose that passing Eastward through the tunnel beneath the English Channel might make me a Frenchman endowed with all the legal trappings of a citizen of that country. It’s simply a preposterous idea.
No, FAR better to establish a definable, recognizable, testable, instant in time where one actually can be legitimately construed as having BECOME a “human being”; and the moment when all the necessary DNA that makes you you is first brought together is the only moment that meets those criteria, and that is conception.
Now too, as you point out, “person” and “human being” are two terms for exactly the same thing in context to speaking of the personhood of human beings; that one is a “person” is inherent in that one is a “human being.” So, when the egg becomes a human being is also when the egg becomes a person endowed by [his] Creator with certain inalienable rights; life chief among them. And science is unequivocal in asserting that the egg becomes a human being at conception, as that is the earliest definable instant where all of the requisite DNA is present together.