human biology - What prevents a pregnant woman's immune system from recognizing her fetus as nonself (and attacking)?

I'm familiar with the scenario of Rh- mother with Rh+ fetus having complications (more so after her first child), but that's not what I'm curious about. I want to know mechanistically why a pregnant woman's body does not display a reaction similar to what we see with rejection from a mismatched organ transplant. On the most basic level, I feel as though the fetus should be recognized as "nonself" due to paternal genetic contribution.

I feel as though if this mechanism could be understood, it could be harnessed for other situations (e.g., organ transplants).