There is not much research on sleeping positions and their safety during pregnancy. One thing we know is that blood flow to the placenta is best when a woman lays on her left side. This is because there is a large vein in the back on the right side of the spine which returns blood from the legs to the heart. When a woman lays on her back, the uterus can partly block this vein, which reduces blood flow to the heart, reducing blood delivery to other parts of the body. This is probably not significant until after about 16 weeks, however.

An interesting study found that women slept as follows:

	Non-pregnant	Pregnant
Back	33%	34%
Side	60%	65%
Stomach	7%	1% before 16 weeks, 0% after 16 weeks

This shows that women naturally will stop sleeping on their stomachs as their uterus grows.

To summarize, it appears the best position is left side, followed by right side. After 16 weeks it's best not to sleep on your back, and you probably won't sleep on your stomach. (One way to avoid sleeping on your back is to sew a tennis ball into the back of your pajamas; this works for people who snore, too).

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