Illustration by Barry Falls

When my first son was born in 1991, the rule was that he should never, ever sleep on his back. Babies aspirate that way, the pediatrician warned.

By the time my second son was born three years later, the rules had changed. Never, ever let a baby sleep on his stomach, the pediatrician warned. It’s a recipe for sudden infant death syndrome.

My boys’ births straddled the “Back to Sleep” campaign, which was introduced by the American Academy of Pediatrics in 1992. In the years since then, the death rate from SIDS has been halved in the United States, some studies show, and lots more parents have asked their doctors why their infant has a somewhat flattened bald spot on the back of his head.

There have been other unintended and unexpected results as well. Not only are parents not putting babies on their stomachs when it is time to sleep, they are also not putting them on their stomachs at all (or, more precisely, not nearly enough). In Slate last week, Brian Mossop analyzed growing medical evidence that this lack of “tummy time” has developmental consequences for children that are just becoming apparent.

Mossop writes:

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.

The studies Mossop cites suggest that the less time a baby spends on her stomach, the later the age at which she begins to stand on her own, and that, in turn, is linked to everything from lower I.Q. scores, to performance in phys-ed class, to adult cardiovascular fitness. As Mossop writes of one study out of Finland:

… 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 I.Q. 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.

Over the past few years, therefore, some pediatricians are adding to their advice. Put baby to sleep on her back, they say, but also make sure she spends some supervised playtime on her front. It’s as simple as lying down with a newborn on your chest before they can support their own heads, and surrounding them with toys they can look at and reach for once they can.

If they fall asleep, and when it’s time to sleep, flip them over. Gently. Because the rule “never wake a sleeping baby” never goes out of style.