I think that the correct answer is for the Supreme Court to overturn Roe on the basis that abortion is unconstitutional as a violation of the equal protection and due process rights of people - to wit: the unborn.
The USSC has no power at all to decide at what precise point a fertilized human egg becomes a person. Their previous guesswork on the legality of the matter in Roe will probably have to suffice for now.
The contention that legal person-hood begins [and can be protected by the state] at the instant of conception is incompatible with our principles of common law.
Wait.
Are you arguing that neither the States nor the Federal government can decide when life begins as a matter of law, because of precepts of Common Law?
Or are you arguing that the Supreme Court has no power to make such a decision, and that this is to be left to the States alone to decide?
Suppose the States decide that life begins at the age of reason, and so vote. And this allows parents to kill children under the age of 7. Is the Supreme Court powerless to strike down that law under your theory?