Do you think that personhood is really an 'empirical fact'? How would you empirically measure when a developing fetus or infant's (or toddler's, depending on your view of personhood) brain becomes a person without a value-laden definition? Likewise for temporary or permanent brain damage.

Is personhood really a binary proposition at all, or a matter of degree?

Of course, for almost any non-incoherent definition of personhood, the degree of personhood during the first trimester is roughly nil.