The law should never have gotten itself entangled in this. Under the law, a “person” has rights. But a “person” is not, under all circumstances, a human person. A corporation is a “person” in many senses under the law, having all the same rights as a human “person” has.
When it comes to human “persons”, the law readily allows that a person born naturally is a human “person” in every sense. American law has, by and large until Roe, regarded a human being a “person” from conception to natural death. In its eagerness to allow abortion on demand, the Supreme Court in Roe went into mental contortions without ever really saying when, on the continuum between conception and natural birth, a human being is a legal “person”. The Court admitted that it didn’t know. And because it didn’t know, its rationale went, it wouldn’t protect an unborn child in any meaningful way. We now have an absolute confusion in which abortion can be legal right up until birth. But, as in the Carhart case, if the baby’s body is outside the mother, then it can be prohibited by statute, though not as a consequence of a natural or inherent right. Also, since states are allowed to allow doctors to neglect a child born alive through artificial (abortive) means so that the child dies, that is permissible. However, it is not permissible to allow a child born alive through natural means to die through neglect. But then, if a child is delivered prematurely through, say, a caesarian, they cannot neglect it to its death. Further, states are allowed to declare a fetal death through outside hostile action (say, as a result of an assault on or murder of the mother) a murder of the child that, in other contexts, is not a “person” and cannot be murdered.
So, putting all of it together, (and there are other inconsistent rules) it’s a matter of the mother’s intent as to whether an unborn (or even born alive if the original intent was abortion) child is a “person” under the law, and thus entitled to protection.
So, the Supreme Court, in effect, said “we defer to the mother the decision to confer “personhood” on an unborn child, because we don’t know how to do it ourselves”. Never was there a more craven declaration.
And, of course, “personhood” is deemed to end when a doctor has a particular interpretation of brain waves. Thus, the Terri Schiavo result. So, then, personhood ends when someone other than the law decides it should, and it begins the same way. Other decisions in favor or against “personhood” can reasonably be expected, and can be expected as deferrals to someone else.
The law, of course, could have defined it as the commencement of conception to natural termination of life. Why do that? Just for the sake of simplicity? I think it runs deeper than that. Societies should not formulate their collective mores based on what the law says is legal or not. But they often do. A substantial segment of this society has “learned” that what could be regarded as a “human person” may be regarded otherwise, depending on the decision of someone or other; the mother, a doctor, a probate court. There are so many shifts and changes and inconsistencies in all of it that reverence for human life as such is just lost, and it all becomes relative, based on the supposedly pragmatic judgment of someone or other.
Any human society can design its laws in whatever way it wants. Even the supposedly sacrosanct U.S. Constitution can be changed, either directly or by court interpretation that is left to stand. It seems to me that once absolutes that make sense, even if they are not necessarily scientific in every way, give way, the society is left adrift in a moral sense.
Whether the Founders were Deists or not (and some probably were, while others certainly were not) they still had notions of morality behind what they did. Even Deists have morality. In setting what they thought were absolutes, they openly declared that they were morally based. They, at least, did not think that the law could or should be utterly divorced from morality. If we do not agree with that concept, we do not agree with them. Nor do we agree with the immutability of any law governing conduct.
I think we also need to take into account that there was a tradition that is common to a large number of societies that personhood was conferred on a child some time after birth. This probably was a result of a high incidence of neonatal mortality.
As far as the Law “should never have gotten itself entangled in this,” I’m afraid it has no choice. There is no other institution to do the job, in fact it is the right institution.
I think you do a good job of outlining the confusion that surrounds the matter. There’s a lot of well-meaning, highly moral people in the U.S. legal system and elsewhere who have struggled with the question, and the result reflects how otherwise intelligent people come to different conclusions.