My 15 week old baby has developed a flat head at the back, presumably from spending so much time on his back. I've heard it said that this will resolve itself over time but I also know some parents put their babies in expensive helmets to sort it out. I try my best to get him to spend time on his tummy during the day but at his age there is only so long he will tolerate it. I really don't like the idea of putting him in a helmet. Does anyone have experience of the flat head problem resolving itself over time (or not)?