Since you offered the following so assuredly, you must also know that this is not the case in all states and in fact a few recognize the unborn all the way back to conception. But you made a nice broad stroke for defense of the dehumanization ... "In the second semester the fetus is a provisional 'person', being the chattel of the woman as though it was one of her children. But in the case of a second semester fetus, it is only a crime against the woman and not society at large." [Was that the case in the Scott Peterson trial? ... very deceptive of you, turtle.]
Actually, it was the case in the Peterson trial. It has been traditionally used for "double murder" when someone killed a women in the second semester. Because the woman is also dead, the state is essentially asserting the right to the child's life on her behalf since she obviously cannot testify. In cases where a second semester fetus was killed but the mother lived, the killer was only liable if the mother brought charges -- the state had no legal interest.
Again, I'm not taking a position here, I am pointing out the English Common Law that defines how our laws are interpreted. I fully agree that the legal reasoning in Roe vs. Wade was specious and incorrect, but we need to understand that even without that specious reasoning the Supreme Court would have a very hard time banning abortion outright due to the extremely deep precedent in the Common Law that asserts the state has no interest. In this sense, the best and correct ruling would be to defer this decision to the individual States, a number of which operate under other Common Law systems (notably Spanish and French) in addition to English Common Law, with different interpretations of the legal precedent. The Federal government had no legal interest in precedent, so they invented a legal interest to keep control. The States have far more flexibility in this matter than the Federal government. We have to be careful of what we ask for, or we may find ourselves in a much less tenable position than we are now -- unintended consequences due to operating from a naive legal perspective.
I would also point out that "conception" is as legally ambiguous as "birth". These terms do not have strict definitions and are therefore fuzzy at the edges -- see "partial birth abortion" for an example of people exploiting the boundary area. This will always be a contentious and fuzzy issue no matter where you draw the line for legal protection. As I stated, the common law gives three levels of protection from conception to birth: no legal status in the first semester, legal chattel of the women in the second semester, and full independent legal protection at birth.