My midwife says there is no reason to not lay on your back so long as you are comfy, and that that advice is a little old fashioned. I still lay comfortably on my back at 36 weeks, I've never had any dizziness or anything like that. In fact, the only time I have had issues has been a few times on my side, but generally I can lie however I want. I love not having to worry about how to lie (except I WISH I could still get onto my stomach!! ) AS long as you are comfy you'll be fine, your body will let you know pretty fast if it's affecting you.