I agree with your “if”, but I agree with Scalia that finding that “if” would be no less an invention than was finding the “right” to abortion. Either one may fit a modern desire of some but was not likely put there in the writing of the Constitution and it’s amendments. It is to me not a question of what “ought to be”, but instead what is really faithful to the Constitution in it’s “original intent”.
That is the crux of the matter. Would the founders have considered the pre-born to be human and, therefore, granted rights (particularly the right to life)? I would not consider this would be an "invention" of a new right and outside the scope of a SCOTUS ruling. I would consider it a clarification of an existing right.