In theory you could have your partner become your surrogate, guaranteeing your full rights to the child no matter what.

We live in a world where prenups are thrown out because the judge just doesn't think they give enough money to the woman. Contracts don't mean anything if the state doesn't think they should, and the state has a pretty clear opinion that men are obligated to serve women, and never the other way around. I would be shocked if there was any way, regardless of how many stacks of contracts she signed, that a mother living with her biological child would actually be prevented from getting at least partial, it not full custody if she actually wanted it. Especially if she was willing to claim abuse.