Prenatal care like vaccinations more than pay for themselves at the societal level. I can see the argument for keeping the government out of healthcare, but this really does fall under moral hazard.

Also, if a female can go from no prenatal care to prenatal care then there going to swap from one to the other when they decide to have kids which would distort the market. In the end it's the same reason you can't get insurance with and without support for specific diseases.