Sleeping on your left side during pregnancy
Q Why is it better to sleep on your left side when pregnant?
A The answer to this question lies in the anatomy of the abdomen, which has to harbor the ever-increasing uterus as the baby grows. The Vena Cava is the main vein that drains the entire lower half of the body. Anatomically, it lies just to the right of the midline--just on the right side of your spine. As the baby gets bigger, certainly the heavier uterus, lying flat on the Vena Cava will (like stepping on a garden hose) obstruct flow up towards the heart. The drainage of the lower half of the body becomes sluggish, which not only increases the swelling of your ankles, feet, and legs, but will also impact on hemorrhoids as well. And decreased return of blood flow to the heart will cause hypotension (lowered blood pressure) down the line and resulting diminished arterial blood flow to the uterus, placenta, and baby.
Sometimes this hypotension is evident when a woman has an ultrasound, during which she lies flat. One of the symptoms of hypotension is nausea that will accompany the light-headedness. In answer to your question, lying on the back is the worst ossible position in the third trimester. Lying on the right side is better than lying on your back, but lying on your left side is the best of all, because this is the position which will have the least amount of weight upon the Vena Cava.