I have read thats its from 12weeks on wards. The best side for sleeping is your left side.
I am mostly a side sleeper, but, occassionally, i do tend to be more comfy on my back, in which case, i stay like that.
I do find however that having an extra pillow to cuddle helps me to stay on my side, or, if am am wanting to sleep on my back, i put it under my right side.