However, in the holistic nutrition world, many practitioners take the Maimonides (Medieval Philosopher and Physician) health principles to heart. Maimonides recommends sleeping on the left side for digestive health
A person should not sleep on his front or his back, but rather on his side, at the start of the night on his left side and at the end on his right. He should not sleep right after eating, but should wait three or four hours. One should not sleep during the day. (Book of Knowledge, Chapter 5, Verse 5)
The unsubstantiated reasons have to do with the fact that the liver is on the right side, so either (1) the liver would be free of added pressure and would be capable of doing all the work it has to do while you sleep, or (2) the liver would be resting on your stomach and therefore keeps the digestive system "warm" and functioning properly. And for pregnant women, the liver won't be resting on the uterus or major blood vessels.
So I was really interested to see an Observational study in the British Medical Journal about sleep positions and stillbirth. The results found that
Women who slept on their back or on their right side on the previous night (before stillbirth or interview) were more likely to experience a late stillbirth compared with women who slept on their left side...The absolute risk of late stillbirth for women who went to sleep on their left was 1.96/1000 and was 3.93/1000 for women who did not go to sleep on their left. Women who got up to go to the toilet once or less on the last night were more likely to experience a late stillbirth compared with women who got up more frequently... Women who regularly slept during the day in the previous month were also more likely to experience a late stillbirth than those who did not. ( Source ).
This was a retrospective study so keep in mind that it's difficult for a grieving mother to actually remember how she slept and she may also be looking for a reason to explain the stillbirth, grasping at straws. In addition, it is an observational study and therefore cannot point to cause and effect. To do that, researchers would need to prescribe sleeping positions (back or side) to similar groups of pregnant women, and I suspect that getting approval for that type of study might be difficult due to ethical reasons.
There is an accompanying editorial , "Should Pregnant Women Sleep on the Left?" The authors question whether this association between sleep position and stillbirth is rooted in any science, or caused by chance, bias, or reverse causation (i.e. the stillbirth makes it more comfortable to sleep on their back). They conclude that further research is needed before a recommendation is made on sleeping positions, although they quote a study that observes that pregnant women may naturally choose a left leaning position anyway (Darwinism/Evolution theory at work??).
Although we cannot make any clear public health guidelines at this point, the public health clinician in me can already see the social marketing in the works, on the literal heals of the "Back to Sleep" campaign that has saves many babies from SIDS: "Back to Sleep for Babies, Not for Moms"