Plagiocephaly - Flat Head: I've been meaning to do a video on this for some time. There is not a lot of information on the 'side effects' or concerns for short and long-term development. I believe that the changes in ones ability to initiate and establish functional developmental milestones can be moderate to severe depending on the overlap of other complications that have occurred in your child's life.
The most obvious structural change is the placement of the flat spot on the skull. During more typical development, your baby will be rolling around from their backs to their tummies by 4.5 months. These actions creates a minor centrifugal force to add to the skull's presentation.
When your baby does not roll around or introduced to 'tummy time' in various positions, a flat spot can start to appear. Years back, this only happened due to complications in the labor process. A new study is suggesting that 61% of all babies are showing flat spots!!!
One of the biggest complications to the human body's ability to move shows up in the back of the head. The rectus capitis posterior minor and major (short muscles in the back of the head - see the second picture) are now able to contract. With typical skull formation, one is not able to contract those muscles with free will. We would use longer muscles, splenius capitis and cervicis to perform this task (third picture).
The second biggest observation would be the countering of the head from the spine. If you look at the next two pictures, the head counteracts with spine and body movements in a 'floating' action.
When a flat head presents to the spine, the fulcrum point of the spine changes creating more of a 'draw bridge' manipulation or backwards head drop. See in the last two drawings.