A woman has to be on her back and underneath to get pregnant? False.
There is no scientific evidence to recommend one sexual position over another for conception. Any recommendations are based on speculation. The body is made to encourage reproduction. The angle of the vagina is designed to take semen where it needs to be, and cervical mucus helps sperm along [source: Santa Monica Fertility]. A woman doesn't have to get into a headstand after sex, either, as some suggest -- sperm are chemically motivated to get to her egg.
Timing is far more important to conception than sexual position. So enjoy your time in bed (or on the floor, or in the car) however you like it.