In the third trimester, you should be sleeping on your side. Lying on your back puts too much pressure on your vena cava (the vein that transports blood from the lower body to the heart), and lying on your belly is obviously out of the question. Lie on whichever side feels best (and switch throughout the night), though experts recommend lying on the left side to allows optimal blood flow to the placenta.
You’ll probably find side sleeping is most comfortable when you bend your knees and place a pillow in between them. You might also want to place a pillow between the side of your belly and the bed, and behind your back for support. A body pillow (or just a collection of strategically placed head pillows) can also help you find a comfortable position. Tip: If you want to try out a body pillow before investing, ask around to see if a friend has one to lend for a night or two.