Getting comfortable for sleep isn’t always easy when you are pregnant. In early pregnancy, tender breasts and a growing womb often means that lying on your tummy is uncomfortable, and in later pregnancy often every position feels uncomfortable!
Probably the best sleeping position is on your side, preferably the left. This is good because it makes it easier for the heart to do it’s job of pumping blood around you and the baby. Lying on your back in later pregnancy means that the weight of the baby presses against the large veins which carry blood back to the heart. This can make you feel dizzy and faint. However, in a normal healthy pregnancy, it is very unlikely that you would cause any harm to you or the baby if you slept on your back with your head supported by pillows for parts of the night. If you stayed on your back for a long time, you would wake feeling uncomfortable and change position long before any harm could be caused.
However, if you are used to sleeping on your back a lot, it would seem sensible to try to get used to side sleeping during the earlier part of pregnancy. As you get larger, a pillow under your tummy, and one between your knees can make you more comfortable. If you really can’t get comfortable this way, sleep on your back, but put a pillow or cushion under one side of your body to tilt you one way or the other slightly. This will take the pressure off your large veins and also your lower back. 1
1 Henderson and MacDonald 2004: ‘Mayes Midwifery’