When should you stop sleeping on your back and stomach and start sleeping on your side?

Well you should stop sleeping on your stomach when it is no longer confortable for you, which is usually when you start getting a belly. As for your back everyone will say you shouldn't, but thats the only way I could sleep when I was pregnant. So go with whats the most comfortable for you!

I read between 4-5 months. By then the uterus is heavy enough to cause pressure on main ateries when you are laying on your back. I am on pregnancy number three and I started sleeping on my side around then each time.

By about 20 weeks they say to not lie on your back. But, if it's is uncomfortable before then, then lie on your side sooner.

also, if you wake up and have rolled onto your back - don't worry, just roll to your side again and you'll be fine.

ASAP, that way you will be in the habit of sleeping like that when your belly is big and its uncomfortable to sleep any other way!