Just had to add my little bit while preg with my DS I couldn't sleep on my left side after hearing my hip go "POP" after sitting down one weekend around 7wks preg. I still can't lie on my left side, even though he's over a year old now. I was told by my physio that I should just sleep in whatever position was most comfortable for me, and if it got too uncomfortable on my back or on my right side to prop myself with pillows under my left hip and shoulder.

I have never heard of not sleeping on the right side though.