A Fairly Common Condition Among Infants
"Plagiocephaly", also called 'deformational plagiocephaly' or 'flattened head', is a condition in which an infant's head is flatter on one side than on the other. This is typically caused by sleep positioning, although it can also be caused by numerous other health and developmental conditions. For example, an infant with a sensory processing disorder might prefer to feel the sensation of their cot sheets against one cheek but not the other.
The condition is most commonly found in infants under the age of eight months, the reasoning being that these infants are yet too small to be able to crawl, so their heads remain in contact with the cot mattress and cot sheets for many hours at a time.
Pediatric medicine has seen an increase in occurences of plagiocephaly since 1992, when the American Academy of Pediatrics began their "Back to Sleep" program, recommending all parents place their babies to sleep on their cot sheets on their backs instead of their stomachs. While this cut the SIDS rate dramatically, it's caused a huge increase in the number of cases of plagiocephaly, and there is some debate as to whether it's had a negative effect on infants learning early motor skills.
As newborns sleep for many hours at a stretch and babies under a year old typically get anywhere from twelve to fourteen hours of sleep daily, their skulls are resting for hours on their cot sheets, resisted by the firm cot mattress. A firm cot mattress is necessary to lower the risk of suffocation or injury, but the pressure from these mattresses is the direct cause of plagiocephaly. Parents are advised to give their infants plenty of "tummy time" to offset all the time spent lounging on those cot sheets.
No Need for Treatment
Due to the developing state of your baby's skull, if you notice flatness on one side of the head, it's likely that the condition will cure itself. You can help by changing the direction your baby faces while he sleeps, placing special support pillows under or on top of his cot sheets beneath his head, and alternating which side of his cot is the "head" and which is the "foot". In other words, if you put him on his cot sheets with his head to the North tonight, place him down tomorrow night with his head to the South. This way, if the plagiocephaly is due to him facing a favorite direction (as is often the case), he'll naturally have to turn in a different way.
Examine your baby's cot sheets, too, and get rid of any with worn areas beneath his head. Cot sheets made from "fuzzy" fabrics will show wear in the areas he's frequently resting. By disposing of these worn cot sheets, you're reducing the risk of them wearing through, which could present an aspiration hazard while he sleeps.