The position does not matter in early months of pregnancy. It is preferred to sleep either on left or right lateral position. But once you go to sleep, you have no control over the position. Sleeping on your back may compress the abdominal vena cavae. From ages that has not created much problem.
there is no "best prayer" when you are speaking to God he doesn't care which prayer you are praying, he just wants to hear whats on your mind. say whats on your mind and that is the best prayer you can say.