Why would a miscarriage result in an investigation anymore than an abortion would? If you're granting a woman the sole right to her body and to decide the path of her fetus, then she has the right to miscarry, stand on chairs, whatever. It's her body, right?
It seems you're granting status to the fetus, rather than to the mother.
If you don't grant status to the fetus, than it cannot by definition be murder. You can't murder a thing, you can murder a person.
If you are going to charge someone with murder (rather than another form of violent charge) with all that entails in our legal system it has to be for killing a person.
Its not Anti-Choice to treat a fetus like a fetus and not charge someone with murder for causing a miscarriage.
Making a choice does not involve rewriting reality for everyone else. Unless you are going to declare a fetus a person (and all that entails) you can't charge someone with murder for causing a miscarriage.
There is already penalties and punishments in place for causing a miscarriage, murder is not a general catch all crime to be bandied about.