What would happen if a 18 year old got a 15 year old pregnant but she was 16 when she had the baby?

It's irrelevant when the baby is born since it's the sex that is illegal. The sex could have been legal if there was close in age exemptions to the law in your state, otherwise it was statutory rape since she is under the age of consent. Anyone can report this and not just her parents. A judge can also find the age of consent should be higher and raise it to 18 because of the pregnancy and age difference.