Sleeping position.

I know the "best" way to sleep is on your left side, but I sleep the best on my back. I try to sleep on my side and always wake up on my back even with a pregnancy pillow. I'm 20w2d with my first so I was wondering if I'm going to do some kind of harm by sleeping on my back. Of course, I am sure my back will ache, but I thought I read somewhere that sleeping on your left side ensures that the blood flows through the placenta freely. I just want to make sure that I'm not restricting anything essential by sleeping on my back. I hope I don't sound too paranoid here. Haha!