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).
Many pregnant women are comfortable sleeping on their stomach during the first week of their pregnancy, especially if they usually sleep on their stomach. But it can become uncomfortable once your uterus starts expanding and you begin to feel like you are carrying a large beach ball on your stomach.