It goes both ways. But mum is the one to get stuck once she realizes she expected maternity+relationship to be different. Dad can leave, they're definitely more prone to it than mums.
Girls my age just reached that phase of lowering standards for a partner just to have children and it's scary. We had amazing relationship from day one and even we are fed up with each other's shit bc kids are exhausting. People who didn't get a good start are off to trauma for everyone involved. Especially the kid obviously.π