Wow my body must completely suck lol, because I've lain (Uh, it's lain, right?) on my back for however long I've wanted this whole pregnancy and never had an issue. I actually only get a bit short of breath/my chest feels constricted, when I first lie down, and that is on my side. I say sleep how you're comfortable, your body will tell you if you're doing something wrong. I too was a bit worried when I first hear sleeping on your back was apparently bad...then I realised, it actually didn't do anything to me, and I sleep/lie however I feel like. At this stage whatever is the most comfortable wins for me. As a PP said, I actually get more leg cramps etc lying on my side.