I had to sleep on my back my entire pregnancy with my son because side-sleeping gives me horrible hip/leg pain in late pregnancy. When I started to get big during the late second trimester, I would prop my head and back up on pillows and this helped quite a bit with the discomfort of back sleeping, but I was never able to side-sleep and DS was born 8 lb 11 oz, very healthy. I was worried about my need to back-sleep, and asked my doctor about it, and she was not concerned.