According to WebMD, it is not recommended for a pregnant woman to lay on her stomach, as this puts unnecessary pressure on the fetus and reduces blood flow. However, shifting positions during sleep is unavoidable and not something to be concerned about, as the problem usually fixes itself.
Furthermore, WebMD recommends that pregnant women sleep on their left side in their third trimester. This allows better blood flow and reduces the pressure the uterus places on the liver. Positions that maintain the curve in the back are also recommended. Firm mattresses and box springs are helpful, although it is largely a matter of comfort.