Swaddling, the practice of tightly wrapping a baby in a light blanket, is an age-old technique that not only calms babies but makes them portable. The technique is growing in popularity in the United States as a result of a best-selling parenting book that teaches frazzled parents how to soothe crying babies.

This month, the medical journal Pediatrics has weighed in with a commentary, noting that there is a wrong way and a right way to swaddle.

The right way has a seemingly wondrous effect. Fussy babies, with their arms wrapped tightly to mimic the effect of the mother’s womb, stop crying almost instantaneously. The technique has been popularized recently by pediatrician Dr. Harvey Karp, author of “The Happiest Baby on the Block.” But several medical studies, including one report last year in The Journal of Pediatrics, also show that swaddling can decrease crying time. Other studies show it prolongs sleep and even reduces a baby’s risk for sudden infant death syndrome, or SIDS — but only if the baby is on its back. Swaddled babies placed on their stomachs are at higher risk for SIDS, as noted in a review article published in Pediatrics last October.

But there is a wrong way to swaddle. The wrong kind of wrapping can affect a baby’s hip development and increase risk for developmental dysplasia of the hip, a relatively common problem that, if not caught early, can lead to premature joint problems and chronic pain in adulthood. Hip dysplasia as a result of swaddling typically occurs in cultures that bundle their babies for portability, sometimes wrapping their legs and attaching them to a cradle board for carrying.

But the authors of the Pediatrics commentary question if the rising popularity of swaddling in the United States may lead to an inadvertent increase in hip dysplasia among children here. “Although we appreciate that swaddling may sometimes be an effective technique to decrease crying and promote sleep in newborns, there is a concern that it may lead to an increase in hip dysplasia,” wrote the article authors, from the department of orthopedics at Children’s Hospital in Boston.

Parents who swaddle shouldn’t panic. Hip dysplasia is easily corrected when caught. The real message is that swaddlers should make sure they are doing it right and discuss the correct technique with their pediatrician. Videos from “The Happiest Baby on the Block” series also demonstrate correct swaddling technique. The key is to leave plenty of room for baby’s legs to move around. And a simple trick — adding a second diaper over the first, before swaddling — easily alleviates the problem because it pushes a baby’s hips in the right position. In fact, a nationwide program to improve swaddling technique and promote wider diapers in the late 1970s resulted in a five-fold reduction in hip dysplasia, according to the Pediatrics article.

Dr. Karp compares swaddling to car seats — something parents have to learn to use the right way.

“Babies have an on and off switch for crying, but like any other reflex it requires very specific technique,” said Dr. Karp. “For the last 30 years pediatric books have talked about swaddling, and rocking and white noise and all that stuff, but they never taught parents how to do it. These techniques have to be taught very specifically.”