Just to throw another wrench in the works...
My best friend is an OB nurse and she says that instant, overwhelming, amazing, I Would Kill For This Child love that hit me the moment Sumo was born is, in no way, a universal experience. She sees many, many mamas who are borderline indifferent to their new babies or see them as a nuisance from the start.
So their "mama gut" or "instincts" would be a highly unreliable well to draw from in caring for their children.