The most dangerous drug in the world: 'Devil's Breath' chemical from Colombia can block free will, wipe memory and even kill. 1  Scopolamine often blown into faces of victims or added to drinks.
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).
Women often experience insomnia during pregnancy, especially during the third trimester. Reasons include emotional stress, physical discomfort and anxiety about not being able to sleep. I recommend caution with sleep aids, prescription or otherwise, because most haven't been shown to be safe during pregnancy; however, there are many effective nonpharmacologic ways to improve sleep quality and duration.