So the murder of a pregnant woman carrying a 38-week-old fetus is only one murder?
That's a difficult question to answer.
The argument of "when a person becomes a person" will never be settled - even if we could pinpoint the moment that "life" begins - because people will always be on both sides of the debate with decent points. In light of this, I do not think that the problem is being solved the correct way. I don't think it's being solved at all, actually.
Since this is a very complicated issue, I will put morality to the side for now and deal only with legal ramifications if you will allow me. T
The question we should try to answer is not "when a person becomes a person." The definition of a person will waver, and the interpretations become subjective. The question we should ask is much simpler.
"How should we go about solving issues that need to be solved?"
In the instance you suggested, perhaps it would be double murder.
But if the mother was dying, and the only way to stop her death would be to kill the fetus, the government should not interfere with doctors assisting in this "murder." In short, they should be protected in some sense, but a fetus should not have all of the rights of all of our working, tax-paying citizens.
The murder of a man is "bad," unless he is about to kill you. In that case, it is "decent." Similarly, the deaths of fetuses in different circumstances should be treated differently. There's no reason to create some huge, overarching rules that must be followed in all cases. Reality is complex, and so should the law be.