The age-old technique of swaddling a baby, which involves wrapping them in a cloth or light blanket so that only their head is exposed, might actually be dangerous. Although the technique supposedly calms the child and leads to less crying and better sleep, a new study reveals that Sudden Infant Death Syndrome (SIDS) increases when infants are swaddled while sleeping on their stomachs or sides.

The University of Bristol researchers came to their conclusions through the examination of four studies spanning two decades and three diverse geographical areas across England, Tasmania in Australia and Chicago.

"The focus of our review was not on studies about swaddling - a traditional practice of wrapping infants to promote calming and sleep - but on studies that looked at Sudden Infant Death Syndrome (SIDS)," said Anna Pease from the University of Bristol's School of Social and Community Medicine and lead author of the study. "We tried to gather evidence of whether there was an association between swaddling for sleep and SIDS.

"We only found four studies and they were quite different, and none gave a precise definition for swaddling making it difficult to pool the results," she added. "We did find, however, that the risk of SIDS when placing infants on the side or front for sleep increased when infants were swaddled."

Although the study suffers from some limitations given the lack of a definitive definition of swaddling, the analysis highlights the importance of current advice - which already suggests avoiding placing infants on their front or side during sleep - especially when infants are swaddled.

Among swaddled infants, the risk of being placed in the side position is almost double. Furthermore, the risk of SIDS was higher in infants who were swaddled and found on their fronts. These risks were increased for older infants who were swaddled during sleep, and the study suggests that most babies found on their stomachs moved into this position on their own.

"We found some evidence in this review that as babies get older, they may be more likely to move into unsafe positions while swaddled during sleep, suggesting an age is needed after which swaddling for sleep should be discouraged," Pease said. "Most babies start being able to roll over at about 4-6 months.

"On a practical level what parents should take away from this is that if they choose to swaddle their babies for sleep, always place them on their back, and think about when to stop swaddling for sleep as their babies get older and more able to move."

The findings were published in the May 2016 issue of Pediatrics.