When to stop sleeping on your stomach during pregnancy?

When uncomfortable. You won't really hurt your baby at all by sleeping on your stomach; you won't smash it or anything. Eventually as your belly grows, you won't be comfortable on your stomach and you'll move to your side. The issue is sleeping on your back, and honestly that risk has been exaggerated. Tilting to one side or the other in the second half of the pregnancy is a safe way to get some rest.