Close to half of two-month-olds have a flat spot on the back of their heads, according to a new Canadian study that suggests changes in parents' habits may be a cause.
Although not thought to be medically dangerous, the flattened head shape can become permanent, researchers said - which can have psychological implications for kids as they grow up.
"This is super common," said Dr. Lisa Stellwagen, a neonatologist from the University of California, San Diego School of Medicine, who has studied what's known as plagiocephaly.
"With the Back To Sleep (campaign) and the overuse of car seats, and people not holding their babies like they used to, we've sort of rediscovered this problem with infants' head shapes," Stellwagen, who wasn't involved in the new study, told Reuters Health.
Pediatricians in the early 1990s began telling parents to put their babies to sleep on their backs, in an effort to prevent Sudden Infant Death Syndrome (SIDS), also known as crib death.
That campaign was "enormously successful," Stellwagen said - and the new findings do not mean parents should stop following that advice.
But there are steps they can take - such as holding their baby as often as possible and having "tummy time" when the baby is awake and supervised - to limit skull deformations, she added.
For their study, Aliyah Mawji from Mount Royal University in Calgary, Alberta, and her colleagues examined the heads of 440 healthy infants seen for their two-month well-child visit at one of four clinics.
They found that 205 babies, or about 47 percent, had some sort of head deformation visible to the naked eye. More than three-quarters of those were mild, the researchers wrote Monday in Pediatrics.
Prior studies suggested that anywhere from 3 percent to 61 percent of babies have a flat spot on their head, Mawji and her colleagues noted.
Some research has tied plagiocephaly to delayed crawling or rolling over, but babies tend to catch up by 18 months, Mawji said - so it's the potential for being bullied as a child that's more of a concern.
She said the deformation can be corrected with a helmet - but those typically cost $1,000 to $3,000, so they should only be a "last resort."
For young babies, treatment for a flat spot looks a lot like prevention, researchers said.
"You want to vary the side of the head that you're putting your infant to sleep on," Mawji told Reuters Health. "If their head automatically turns to the right … what you need to do the next night is turn their head to the left."
Like Stellwagen, Mawji recommended parents keep their baby out of a car seat when they're not driving. She also said they should alternate the hand they hold the baby in while feeding.
Parents might not notice a small head deformation because they get used to how their baby looks, Stellwagen said, so it's important for doctors to take a close look at the skull at early well-child visits.