Then, what constituted justifiable homicide of unborn would be left to the States to define, as any other justifiable homicide is. “I want an abortion because I forgot to use a condom or take my Pill” would probably not pass muster for equal protection, however.
The abortion lobby knows this, of course. That’s why you see them freaking out when a Scott Peterson is up for murder: they don’t want the personhood of fetuses established in a court.
No, but a state that wanted to do so could make the justifiable-homicide rules large enough to drive a truck through. After all, how many pregnancies don't expose a woman to some level of risk? While abortion certainly has risks of its own, trying to put explicit measurements of risk into legislation or--even worse--into a Constitutional amendment seems to me a recipe for trouble.
Besides, I think that it's best to focus first on getting the matter returned to states, especially since one may have some allies in that task who would not support having the federal government force states to take any particular action. Pushing for the total abortion ban first would give up those allies.