In what context? 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, and a woman cannot commit a crime against herself, whereas if you killed a born child it would be a crime against society as well.
No matter what position one has on this issue, the dividing line is somewhat arbitrary and ambiguous -- even "personhood at conception" has this feature. Therefore, any legal tradition on this issue will be a compromise of some type. It is important not to ignore that the Common Law has been refined in application for thousands of years, trying a lot of hard cases and edge cases, and the guidelines that were developed approximate the best guidelines for the largest number of legal cases. As always, there are many unintended consequences were one to change such guidelines, and setting radically "new" ones (every guideline you can think of has probably been tested in this system, at one time or another) by fiat is very likely to end up with a legal situation that is worse than what we have. Heavy-handed messing with the Common Law is likely to have the same consequences as when liberals do heavy-handed messing with the economy.
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.]