My four-month-old nephew has a flat head from sleeping on his back. The pediatrician says it is going to remain that way unless his parents get him to sleep on his stomach. How serious is the "flat head" to development and should it be a serious concern for his parents (physically and aesthetically)?
You ask a great question. The Back To Sleep campaign launched a few years ago has been a tremendous success in terms of dramatically reducing the chance of Sudden Infant Death Syndrome, which 90% of the time affects babies under 24 weeks of age.
Unfortunately, one of the fallouts has been positional molding, a flattening of the back/side of the skull from the pressure from outside surfaces preventing normal symmetric growth. What this means is there are many gorgeous and very developmentally normal infants out there with slightly molded heads. Development is indeed normal and in the vast majority of cases the asymmetry resolves as the infant spends more time upright, rolls over in bed on his/her own and less time sleeping. It only becomes a cosmetic issue when the molding is severe, or when the child is approaching the age of two when the growth of the head is 75 - 80% complete.
In some cases, an x-ray is needed to distinguish this benign molding from a premature and abnormal closure of the sutures called craniosynostosis.
Usually, the advice given is to try to spend some awake time on the belly, not back, to lessen the time with pressure on the back of the head. Rarely, very rarely, is a helmet used to protect the head from those outside forces.
I need to reinforce, however, that sleeping a child deliberately on his belly puts him at an increased risk of SIDS, so if you use the advice about positioning him that way, do so when he is more awake.