Why is my baby's head so flat?

Sleeping position: Often, babies who are spending the most of the day on their backs can develop a flat head. Some babies prefer to lay with the head turned to one side or the other and this may also contribute to the problem. Though it is important to sleep your baby the back, it is advised to offer "tummy time" throughout the day to minimize this process. In may cases, the condition improves with repositioning.

Back to sleep: Pediatricians now universally recommend putting babies to sleep on their backs to avoid sids (sudden infant death syndrome). Since a babies' skull bones do not fuse together until about 15 months of age, they move causing a flattening of the back of the head. This is temporary, as long as you give your baby plenty of "tummy time" when she is awake.

Back to sleep: AAP recommends back to sleep , hence the skull gets flat on the back, more tummy time will help