I think you are overlooking the addiction aspect of the problem. Addicts will sell their kids to get their next high, as getting high is the only thing that matters, with some overdosing and killing themselves. They can't even fathom the damage it's doing to their bodies, much less what it is doing to the baby inside their bodies. I am not going to sit here an educate you on what drugs do to addicts brains, how it destroys their reasoning skills, and how it effects judgement. But I urge you to go read up on it, so you can understand that charging an addict for murder after a miscarriage makes zero sense and will accomplish nothing.
You mentioned your wife, and the miscarriage' she had at 14 weeks. Hypothetically, what if those test came back and showed that something your wife ate/drank/took caused the death, something she didn't have to take/eat/drink, but chose to, lets says she chose to have some wine, and that's what did it, you would be okay with her having the book thrown at her? Keep in mind, choosing to have a glass of wine or eat something is a easy decision to do or not to do, unlike the decision a meth addict would have to make, as the addiction/drugs control them.