Since the mid-1990s, pediatricians have urged moms, dads and caretakers to place sleeping infants on their backs to help reduce the risk of sudden infant death syndrome, or SIDs.
Cases of unexplained death have since dropped by more than half, but a far less dangerous side effect has also cropped up: A growing number of babies now have "positional plagiocephaly," or flat spots on the sides or backs of their heads.
A new study in Canada, published in the journal Pediatrics on Monday, found that more than 46 percent of 2- to 3-month-old babies may have some form of the condition, although most cases are mild.
Flat areas on the backs or sides of babies' heads are typically caused by pressure on the bones of the skull and can develop in the first few months of infants' lives. In some cases, the change in head shape is so subtle it is difficult to spot, in others it is quite clear.
"The suggestion is that there has been an increase in the development of 'positional plagiocephaly' since the 'Back to Sleep' campaigns," study author Aliyah Mawji, a researcher and registered nurse with Mount Royal University in Alberta, Canada, wrote in an email to The Huffington Post.
She warned that there is no earlier data with which to compare the new numbers. Therefore, the apparent increase in flatness of babies' heads may be due to the fact that people are generally more aware of the condition now than they were before.
The good news, experts say, is that the flat spots are generally harmless. Mawji said there is some indication that children with positional plagiocephaly have mild developmental delays, but that those typically disappear by 18 months.
"There are no functional problems that I know of, except for a distorted head," said Dr. S. Anthony Wolfe, head of plastic surgery and director of craniofacial surgery at Miami Children's Hospital, who did not work on the new study, but has reviewed it.
The condition can be treated with specialized helmets that babies wear, typically after 6 months of age, but Wolfe stressed that parents would typically only consider that option in more severe cases and for cosmetic reasons. "Some commercial makers of helmets insinuate that if you don't treat it, you may have some [jaw] dysfunction," he said. "But it's really for the head shape and the head shape only."
When it started, the primary focus of the "Back to Sleep" campaign -- now known as the "Safe to Sleep campaign" -- was to ensure babies were placed on their backs during naps and at night in order to reduce risk of SIDs. Although it made a significant dent in the number of SIDs cases, in recent years, the decline has stalled. Newer campaigns focus on a broader range of safe sleeping practices, like using a firm mattress and avoiding soft bedding in a baby's crib. Overall, SIDs is rare, affecting just over 2,200 babies per year in the United States, according to the Centers for Disease Control and Prevention.
As for flat spots, Mawji said parents can prevent them by switching the side of the head their baby puts pressure on when sleeping. The American Academy of Pediatrics also encourages parents to make sure their babies get lots of tummy time, in order to strengthen their neck and shoulder muscles, and minimize the amount of time there is pressure on their heads.
The new study looked at the rates of 440 infants at four community health clinics in Canada, but Mawji said the findings could be loosely applied to the U.S., which is more diverse in terms of culture and socioeconomics.
"My best guess is that my results -- 46.6 percent of infants aged 7 to 12 weeks -- would actually be an underestimation in some parts of the U.S," she said.