Or just let the mother-to-be charge her insurance at hospital rates for all the blood transfusions and other health care she's giving the fetus.

(As a bit of completely unwarranted pedantry — and I'm not a lawyer — most crimes in the US and other common law countries have a mental component (mens rea). This means that e.g. to be guilty of manslaughter you must have chosen to do something willfully harmful or at least unacceptably dangerous, such as attacking someone or driving drunk. So fetuses and babies cannot be guilty of those crimes. Of course, the "charge your insurance" thing probably doesn't work either.)