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Putting infants to sleep on their backs, recommended since the early 1990s, has helped reduce the prevalence of sudden infant death syndrome. But because of concerns that the practice might cause delays in motor development — as measured by the age at which babies roll over — parents were encouraged to give their infants “tummy time” when awake to help build upper-body strength.

Now a new study, published in May in the journal Early Human Development, suggests that tummy time may be irrelevant.

Canadian researchers compared 1,114 infants born from 1990 to 1992, just before the “back to sleep” campaign began, with 351 infants born 20 years later. They found no difference between the two groups in the age at which prone to supine or supine to prone rolling began, or in the order in which those behaviors appeared.

They were not able to measure the effect of “tummy time,” but they note that it is not known how many parents consistently use the procedure and that, anecdotally, most who do find it difficult to keep their babies on their bellies for any length of time.

Whether tummy time helps or not, said the lead author, Johanna Darrah, a pediatric physical therapist at the University of Alberta, “the back to sleep campaign has not adversely affected motor development. Motor development happens.”