Is it okay to sleep on your stomach while your pregnant or should you sleep on your back?

The best position is to sleep on your back, but tilted to one side with a pillow under your back. If you lie flat on your back in late pregnancy, your womb can press the blood vessels going to and from the heart (inferior vena cava and aorta respectively) making you feel faint and unwell. Lying on your stomach is OK in early pregnancy, but most women find it uncomfortable after mid-pregnancy.