I can't seem to Sleep on my back or left side for that matter, so my right one gets soooo sore. My DR Told me either side was fine although left is preferred but I'm fine. He said really any way I can get comfy. Pillows, pillows and MORE pillows is all I can tell you. Lol
You don't want to sleep on your back. The baby will put pressure on your uteran artery and that's when you can pass out. Its not recommended. Get a pregnancy pillow it helps and sleep on your left side as much as possible.