Swaddling, as a form of restraining infants' movement, was used almost universally before the 18th century, according to Thach and his colleagues. But recent studies suggested the practice might lead to breathing problems and even death because of the pressure of the swaddling cloths against the infant's chest wall. There has also been concern that the practice can cause problems in hip joint development, or lead to overheating.

Thach and his colleagues investigated the legitimacy of these concerns in a study of 37 infants, who ranged in age from 19 days to about 6 months. The infants were observed as they took short naps in a laboratory setting.

Overall, 33 of the infants, including seven who usually slept on their stomachs, were able to fall asleep within 30 minutes of their swaddling, the investigators report in the September issue of The Journal of Pediatrics. The infants who resisted the swaddle, meaning they did not fall asleep within a half hour, were about one month older, on average, than those who accepted the swaddle.

This suggests that "early introduction of swaddling may ensure greater success," the researchers speculate.

Also, when the swaddle was progressively tightened to the tightness of traditional swaddles, the infants did not show any great increases or decreases in their heart rate, or their number of grunts or sighs, study findings indicate. Their oxygen levels also remained unchanged during the tightening.

Yet, infants did have slight increases in their breathing during the swaddle tightening experiment, the researchers note. "This suggests that the infant may be compensating for some degree of chest wall restriction and/or decreased lung volume," they write.

Finally, the infants were able to spontaneously flex their hips from 45 to 60 degrees while swaddled, and they only experienced a two-degree increase in skin temperature.

The infants were wrapped in swaddling cloths that were "looser on the hips than the traditional swaddle," Thach explained. Also, the swaddle cloth was thin so "overheating doesn't seem to be a problem," he said.

Swaddling during the first year of life is still common in Eastern Europe, the Middle East, Asia, South America and among Native Americans in North America, but it is rarely used outside of hospital nurseries in the United States, the researchers note. Cultures that practice swaddling, with infants normally sleeping on their backs, have a lower incidence of SIDS.