I'm trying to switch from a stomach sleeper to a back sleeper because I am pregnant/advice?

Try side sleeping. I could explain a physiologic reason to avoid both stomach and back positions. However, more simply, I tell my patients to try to sleep on their side, if they wake up on their tummy or back, simply roll to side. Your body is pretty smart and when it is the best time to avoid stomach or back positions for the baby's sake it will typically become uncomfortable for you to be in those positions too.