The condition in infants known as plagiocephaly, or flat-head syndrome, has an unusual medical history.
Some cultures, including ancient Egyptian and Native American societies, intentionally molded a baby's soft skull into a preferred shape using boards or bands worn around the head, according to Comer Children's Hospital at the University of Chicago.
In 1992, because cases of sudden infant death syndrome (SIDS) were rising, the American Academy of Pediatricians issued guidance that infants always sleep on their backs, not on their bellies. [That's Incredible! 9 Brainy Baby Abilities]
This "Back to Sleep" initiative proved successful, and the number of SIDS cases dropped by more than 50 percent, according to the National Institutes of Health.
(The initiative, now renamed "Safe to Sleep," has been expanded to include other sleep recommendations, including no pillows, no plush toys and no sleeping with adults.)
But the campaign had an unintended consequence: Plagiocephaly cases rose dramatically after 1992.
A 1996 study from the journal Pediatrics found the annual incidence of plagiocephaly in the two years after the launch of the "Back to Sleep" initiative was more than six times higher than the annual incidence in the 13 years before 1992.
It was soon confirmed that the skulls of infants who slept exclusively on their backs were noticeably flattened.
There are other risk factors for plagiocephaly beside sleep position (sometimes referred to as positional plagiocephaly). Children from multiple births (twins, triplets, etc.) are at higher risk due to in utero crowding.
Premature babies, who are typically born with softer skull bones because these bones harden in the last few weeks of pregnancy, are also at higher risk of plagiocephaly.
And infants who keep their heads in one position because of a car seat, neck stiffness (torticollis) or bottle position have higher rates of plagiocephaly.
Mild cases of plagiocephaly are considered temporary, minor cosmetic issues that either resolve themselves or are easily treated by shifting the baby's sleeping or resting position.
In some cases, an infant with plagiocephaly can be treated by wearing a helmet that inhibits growth in the prominent areas and encourages growth in the flatter areas.
A 2011 study from the journal Otolaryngology-Head and Neck Surgery analyzed the prevalence of plagiocephaly among teenagers born after the 1992 introduction of the "Back to Sleep" campaign.
The study found that the prevalence in teens was just 1.1 percent, "significantly lower than the 20 percent to 48 percent prevalence found in infants in previous studies, suggesting most children will outgrow the condition without intervention," the study authors wrote.