Personhood has not been defined by science, nor do I think it can be.
Whether or not “personhood” has been scientifically defined, what’s the difference between being human and being a person?
Before Roe v. Wade there was never a personhood argument; it was only introduced afterward as a canard to make the decision to have an abortion a little more palatable. The same thing happened with Dred Scott. “He’s not a person,” they claimed, “he’s black. He’s not a person, though he’s a human technically; but that’s just a little detail. It’s not significant.”
For your thoughtful consumption:
“Personhood is not a separate, added quality that gives human beings significance. It is a property that is essential to being human. Humans are personal beings. It’s part of an inseparable package. For example, an essential part of being a sphere is to have volume. A sphere without volume is not a sphere, but a circle. All spheres have volume. In the same way part of what being human entails is being a personal being.”