From a logical argument, why would a drunk driver who kills a mother and 12 week unborn baby by subject to two murder counts in some states?

Seems horribly inconsistent.

Would Pro-Choice folks argue that the law is incorrect and the fetus should not be considered in a murder count?

have a difficult time on this topic both with comfort and consistency. Your question is an example of the consistency problem. The current PP gotcha videos show how uncomfortable most of us are with abortion outside of the abstract. Here is my attempt to explain why.

We generally establish our morality - what is the right thing to do - based on how the situation feels to us. That feeling is mostly the result of evolutionary selection. Babies are thought to look adorable because parents who didn't perceive their kids as precious didn't do a very good job of keeping their genetic strain going. The cry of a baby is hard to tolerate because parents who didn't feel compelled to do whatever it takes to address their babies' needs again didn't do a good job having grandkids.

We find murder repugnant not because it never occurred to people that killing each other was a bad idea until Moses brought tablets down from the mountain, but because failing to live together peaceably is a bad survival strategy.

The problem is that these feelings that are at the root of most of our right versus wrong understandings aren't actually reasoned, but feelings that we find reasons to explain or codify.

If you've never done the trolley problem, I encourage you to check it out. Here is a version: http://www.philosophyexperiments.com/fatman/

Without giving it away much, one takeaway is that you pretty much have to be a sociopath to be logically consistent the first time you do this type of exercise. Being logically consistent with what feels right is hard to do because the two criteria have very different origins. Evolution designed us to respond one way. Cold policy logic, which was irrelevant for the hundreds of millions of years of our advanced brain evolution, another.

So back to the topic. I admit that to me and I suspect the vast majority of pro-choicers, abortion does not feel right. Once abstracted away from the repulsive details, I can be okay with it. But I am hard wired by evolution to be shocked and appalled by even the thought once the fetus begins to look like a baby. This is a big reason why PP doesn't want ultrasounds - far beyond their inconvenience. This is why the pro-lifers are ecstatic over the gotcha hidden camera videos showing PP officials speaking casually about fetus organ harvesting. This is why pro-lifers always describe fetuses as babies.

This is why hard core pro-lifers (all the way from moment of conception) can be logically consistent, but the rest of us have a hard time achieving that.

So to your 16 month fetus/baby killed by a drunk driver versus abortion.

It seems to me that the reason we want to call the first murder is because we are really angry about the loss as it seems to the mother. An expectant mother does not regard the fetus as a fetus, but as her growing baby. A person. That's how it feels to us.

On the other hand if she was on her way to the abortion clinic when the accident happened I would have a very different opinion on the matter if I was on the jury. It just wouldn't feel the same even the drunk driver's actions were identical.

I can logically reconcile this by recognizing that the "murder" designation for the first case is really just a shorthand way for us to frame a really big loss for which we feel the need for punishment. While the other is not.

Or so it seems to me.