What if the Court extended the notion of natural personhood to the unborn? Wouldn’t the due process clauses in the 5th and 14th Amendments protect the unborn from being killed?
You are exactly right.
Human being = legal person.
That would be the most perfect conformity with reality, because neither reality, NOR THE U.S. CONSTITUTION, provide any warrant for separating the concepts “human being” and “person.” Reality and Constitution (especially read in its essential context, the Declaration of Independence) find “human” and “person” co-extensive.
Human rights begin where human life begin.
And human life begins at the beginning.
However, we may not be able to get there (politically) all in one jump, since our system of law depends on the more-or-less contemporaneous consent of the governed.
Hence the incremental approach, where each step practically necessitates the next one.