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).
You should stop sleeping on your stomach or back by around 20 weeks of pregnancy. By this point, you may notice that sleeping either way â especially on your stomach â is pretty uncomfortable. The concern with sleeping on your back is that the enlarging uterus can compress the large veins in the back of your abdomen, which can in turn decrease the amount of blood returning to your heart. As a result, your blood pressure can drop, and you may feel lightheaded or nauseated.