In the early 1940s, Dr. Harold Abramson, a New York pediatrician, pored over heartrending reports of babies who accidentally suffocated while they slept. As he reviewed case after case, he noticed that a vast majority of the deaths occurred when babies slept on their stomachs. In a commentary in the Journal of Pediatrics, Abramson suggested that the many case reports of infant suffocation hinted that a newborn’s sleeping position might contribute to so-called “crib death,” later called SIDS. In the following decades, other researchers noticed that SIDS was less common in countries where infants typically slept on their backs. Fifty years after Abramson’s study, the American Academy of Pediatrics formally launched a “Back to Sleep” campaign, instructing parents to put babies to sleep on their backs during their first year. The campaign has been hugely successful: Since it started in 1992, the SIDS rate in the United States has been cut in half.

There’s a drawback, however: Telling parents not to put babies to sleep on their stomachs has scared them away from placing babies on their bellies altogether. And taking away “tummy time,” it turns out, cuts off a pivotal avenue of development. The less time infants spend on their stomachs, the slower they generally are to acquire motor skills during their first year, which means the potential delay of simple feats like lifting their heads as well as more-complicated movements like rolling over, crawling, and pulling to stand. Doctors have hesitated to sound the alarm about this, since children usually walk shortly after their first birthday regardless of how much tummy time they’ve had. But a growing body of evidence now suggests that the timing of the motor-skill milestones that precede walking is crucial and can even factor into long-term health and cognitive ability.

Four years after the Back to Sleep campaign launched, its inadvertent effects started trickling into the clinic. Most notably, some infants had disfiguring flat spots on the back soft crowns of their heads. It took a few years for researchers and doctors to realize that the change in sleeping position also affected prewalking motor skills (whether or not a baby had a misshaped head). Then in 2004, a research team led by Bradley Thach at the Washington University School of Medicine studied the difference in head movements between stomach and back sleepers. Thach showed that babies who spent nights on their bellies quickly developed the brain connections and muscle strength to turn their heads from side to side—one of the first motor-skill hurdles. Babies who consistently slept on their backs, on the other hand, were less likely to have sufficient head mobility at 3 to 5 months.

Next a research group at McGill University, directed by Annette Majnemer, weighed in on the effect of sleeping position on development at six months. They found continuing motor setbacks. A larger fraction of the back sleepers than the tummy sleepers couldn’t roll over, touch their toes, or sit upright with arm support.

Pediatricians have had mixed reactions to these clinical observations. Some have passed off the flat heads as a passing cosmetic issue and the lag in prewalking motor-skill development as inconsequential. Others, including the American Academy of Pediatrics, champion of the Back to Sleep campaign, have seen the head shapes and motor hang-ups as a harbinger of future problems and recommended supervised tummy time when a baby is awake.

Still, according to a study published in the Journal of Pediatric Health Care, 90 percent of new mothers receive the proper instructions to put their newborns to sleep on their backs, but only 55 percent are counseled on the importance of having their children spend supervised playtime on their stomachs. Even when parents get the tummy-time instructions, they don’t necessarily heed them. Instead, they complain that it’s hard to get their babies to play on their tummies. That’s understandable, because before a baby can pull himself up, lying face down often isn’t much fun. Frustrated by their kids’ frustration, parents often give up.

How do we know that the babies who miss out on tummy time are at a lasting as opposed to temporary disadvantage? Looking at data from thousands of people born in 1966 in Northern Finland, a research group led by Charlotte Ridgway at the Institute of Metabolic Sciences, Cambridge, has shown that a one-month delay in infant motor development had the same detrimental effect on how a 14-year-old performs in physical education class as a one-unit increase in the same child’s body-mass index. Using the same Northern Finland cohort, Ridgway and her co-authors also mapped a one-to-one link between the age at which infants stand unaided in their first year—another critical prewalking milestone—and their muscle strength and endurance, as well as cardiovascular fitness, at age 31.

Another team of researchers, led by Graham Murray at the University of Cambridge, has been looking into how early motor lags could affect other parts of the brain, like the areas responsible for cognitive functions. Using Northern Finland data as well as stats from a second group of Brits born in 1946, this group found that the sooner children passed their prewalking motor-development marks, the better the more-complicated areas of their brains performed in later life. Every month in advance of the group average that a child learned to stand on his or her own translated to a half an IQ point increase at age 8. By age 26, early motor developers had higher reading comprehension. And by the time they hit their 30s, they had achieved a higher level of education and scored better on executive-function tasks like categorization—how fast they could group objects of similar shape and color.

What’s the best way to make sure that babies who sleep on their backs get their share of tummy time? Dr. John Graham, a pediatrician and director of the Dysmophology Program at Cedars Sinai Hospital in Los Angeles, suggests parents get started soon after they bring their babies home from the hospital. (The exceptions to the tummy-time rule, Graham notes, are babies with weak or stiff necks, an occasional side effect of birth.) With babies who are too young to support their own heads, parents can lie down and cradle them on their chest, introducing the stomach position in a comforting way. When infants get older and get the head-lifting thing down, regularly placing toys around their field of view encourages them to look around and stretch. It’s pretty basic, really. And if babies cry because they’re not used to the stomach position, stick with it for a bit anyway. The long-term benefit is worth the short-term fuss.

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