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.