I have tried to sleep on my side from about 20 weeks onwards. When I was smaller, I found that I needed to use the body pillow between my legs & under my belly to keep me from rolling over. Now that I'm bigger, I don't just roll onto my back - I actually have to wake up to roll over . It is better for both you & baby to be on your side - the more time you spend lying on your back, the more you are encouraging bubs into a posterior position (yuck!).