From what I've read, it can vary depending on how big you are and where the baby is lying. I think the recommendation is from 2nd trimester but others may know better. If YOU start to feel dizzy or sick, that is a sure sign you should be on your side. Left side is supposed to be better than right side - to do with placenta I think (but my placenta is on my left side so I'm not 100%). Being on your back will get uncomfortable at some stage due to the weight pressing down but then again, lying on your sides will get uncomfortable due to hips and belly too. An uninterrupted night's sleep will become a thing of the past. Just as well we get a gorgeous bub out of all this!