Would an 18-year-old get in trouble if he got a 16-year-old pregnant?
Yes, the 18-year-old is a legal adult, the 16-year-old is a minor. What action authorities could take depends upon the AOC in the state where the minor female lives. The minor's parents or guardian can file a complaint with authorities and possibly civil action as well.
Be advised, that all states have laws pertaining to relationships (sexual and otherwise) between adults and minors. Likewise, all states reserve the right to file charges and prosecute the adult involved in such a relationship without the cooperation of the minor or the minor's parents or legal guardian.
The minor involved in such a relationship would not be subject to any criminal or civil action.