Would that have been the case if Laci Peterson had been three months pregnant rather than eight? My understanding (though I admittedly don't follow these matters closely) is that the key point is whether the fetus could survive outside the mother. Would common law apply if it were indeed a clump of cell (it is, for a certain amount of time) rather than a fetus that almost certainly would have survived independent of the mother.
In that unique situation where you have two lives intertwined, it is impossible to know for sure the right course, IMHO. In some translations of the Bible there is a passage that sets out the penalty for causing a woman to lose a child . . . and it is different from the punishment for murder. I have always meant to find out what the original says, not that I consider that infallible, in any event.
Personally, in this situation, I'm comfortable to let a person decide for themselves what to do.