Ok, so I know I'm probably being a nervous first time mom on this one but DS has a small flat spot on the back of his head. I've seen this on other people's babies. If he sleeps on his back as he's supposed to do then it's pretty unavoidable, right? Is there anything I should do to help keep the flattening to a minimum? I try to put him on his tummy some during the day, buut he doesn't put up with it for very long. Is this even worth worrying about?