>But I think that knowingly causing miscarriage of a pregnancy should be investigated as manslaughter.

A) op didnt clarify who was doing the initiation of a miscarriage. There's unclarified ambiguity.

B) charging manslaughter for one person, and providing medical support for another for the same action of facilitating a miscarriage enters a very real legal discourse know as the entire debate around women's rights. If you would like to know more you can review the legal precedents associated therein.