It is safe to sleep in any position that is comfortable while you are pregnant. This includes flat on your back, on your tummy, on your right side, or even on your left side. It is almost impossible to hurt the baby by sleeping in the “wrong” position. This is a frequent worry that pregnant women have but there is no reason not to be as comfortable as possible when you sleep.
So why do the books say you should sleep on your left side and should never sleep flat on your back? This is the type of information that is technically correct but is not true in normal situations. The explanation requires a review of the anatomy of the blood vessels in the abdomen and their relationship to the uterus.
The vena cava is the vein that returns the blood from the bottom half of the body to the heart. It lies in the back of the abdominal cavity. As the uterus grows it sits above the vena cava in the abdomen. It is possible for the pregnant uterus to sit on the vena cava and put enough pressure on this vein to occlude the blood flow. This can dramatically decrease the return of blood to the heart and decrease cardiac output causing problems for both the mother and baby. When a pregnant women lies on her back this phenomenon is possible. This is the reason many sources recommend avoiding sleeping on the back.
In normal situations this does not occur to the point where it would damage the baby. If a women lies on her back and the uterus interrupts the blood flow to through the vena cava she will feel awful. If the woman is asleep she will wake up. She will feel nausea and dizziness and everything in her being will tell her to roll over or get up. This happens well before any damage can happen to the baby. So if you feel comfortable lying flat on your back you can be assured that there is no decrease in blood return to your heart and no problems for the baby. If you wake up flat on your back and you start to feel sick or claustrophobic, roll over or get up. No harm has com to your baby.
So why do the books say to sleep on your left side and not your right side? It turns out that the vena cava is not exactly in the mid line, it is a bit to the right of the center. It takes less tilt to the left then it would to the right to keep the uterus off the vena cava. Sleeping on the right side is fine and if this is more comfortable there is no problem with this position.
There are some situations when it is not safe to be flat on the back when pregnant. This is when something has occurred to take away the bodies ability to communicate effectively. If a pregnant women is having surgery we always tilt them towards the left side. When a women is under anesthesia the body is not able to communicate to the brain that there may be a problem. If someone get knocked unconscious or is impaired from drugs or alcohol, the body may not be able to communicate a danger effectively. But for most normal circumstances the body is very good at letting the women know when a position is not ideal.
What about sleeping on the tummy, won’t this hurt the baby? The answer is no, it will not hurt the baby, but for most pregnant women this is not a comfortable position. The baby is very well “packaged” inside the body and it is very difficult to hurt the baby from external pressure. It actually would require major trauma to the abdomen to injure the baby. I have seen this type of problem in the case of a roll over car accident and in another case when a women was assaulted with a baseball bat. It takes this type of major injury to the abdomen to effect the baby. Sleeping or lying on the tummy will not hurt the baby.