I may be completely wrong here (and will happily stand corrected if I am!) but i thought that it was more an issue of restricting blood flow to the mother, not the baby, and that if it became too much of an issue you'd become uncomfortable and automatically move anyway.. ITMS!? Omg I just re read that and it makes no sense at all but I'm very tired and can't think! Haha!
I wake up alot through the night because i'm on my back and it's uncomfortable so I move.. But lying on my side is not much better as my hips are playing up too!