During pregnancy, the best sleeping position is "SOS" (sleep on side) as it gives the vest circulation for you and your baby. It also places the least pressure on your veins and internal organs. Sleeping on your left side will increase the amount of blood and nutrients that reach the placenta and your baby. Also its a good circulation  which will help reduce potential swelling, varicose veins in your legs and hemorrhoids.

How Can I Sleep More Comfortably During Pregnancy?

Keep your legs and knees bent, then put a pillow between your legs to relieve the stress on your back.

• If you find having problems with back pain, use the "SOS" position and try placing them a pillow under the abdomen as well.

• If your experiencing heartburn during the nigh you may want to try to propping your upper body with pillows.

• During your late pregnancy, you may experience shortness of breath. Try lying on your side or propped up with pillows.

These suggestions could not be completely sound and comfortable, especially when one is used to sleeping on one's back or stomach, but you can try them out. You may also find that they work. Keep in mind that you may not stay in one position all night and rotating positions is fine.

Source: https://americanpregnancy.org/healthy-pregnancy/pregnancy-health-wellness/sleeping-positions-while-pregnant/