Another important and often cited study: Hills, G.H. and A.G. Chaffe. "Sleeping positions adopted by pregnant women of more than 30 weeks gestation." Anaesthesia, 1994, vol 49, pages 249-250.
In this 1993 study, researchers observed the sleeping positions of 52 pregnant women who were hospitalized at 30+ weeks of gestation against a control group of similarly age-matched women.
Left Sidelying -- 76.9%
Right Sidelying -- 21.2%
Supine (on the back) -- 1.9%
Left Sidelying -- 25.8%
Right Sidelying -- 32.3%
Supine (on the back) -- 38.7%
Prone (face down) --
Their study was focused on aortocaval compression, which they indicate is easy for women to avoid while awake, but not so easy during sleep. They question whether women “naturally avoid positions which might encourage caval compression syndrome.”
I found this study to be quite interesting, but it left me with more questions. The pregnant group were all hospitalized because they were experiencing complications or were very high risk, and oftentimes, physicians and medical staff will actually tell women to avoid supine positioning, restrict their sleeping to left side-lying. Another group to gather data from would be a pregnant women not experiencing complications (but therefore not hospitalized). Also, this study was actually conducted in the early 1990s, and I’m not sure of the spread of information on sleeping positions during that time, but a similar study conducted today would be complicated by the copious information that women gather from friends, books and online – all which tend to staunchly support left sidelying as the ideal sleeping position for pregnant women. So I would ask, are women sleeping on their left sides to “naturally” avoid aortocaval compression? Or because they believe it is the “right” thing to do for the health of their baby?