A baby sleeps in a carseat.Using car seats, swings, bouncers and other carrying devices as sleeping places for infants puts them at risk for injury or death, a new study warns.

“Many parents use sitting or carrying devices, not realizing that there are hazards when they do this,” said study author Dr. Erich Batra, of Penn State Medical Center, in Hershey, Pa.

“Infants and young children should not be left unsupervised (awake or asleep) when using a sitting or carrying device due to the risk of suffocation and death,” Batra said.

Researchers reviewed 47 deaths among children younger than age 2 that occurred in sitting and carrying devices and were reported to the U.S. Consumer Product Safety Commission between April 2004 and December 2008.

All but one of the deaths were caused by asphyxiation. Two-thirds occurred in car seats, and 52 percent of those deaths were caused by strangulation from straps. The other babies died in slings, swings, bouncers and strollers.

The time from when infants were last seen alive to when they were found dead ranged from four minutes to 11 hours, according to the study published April 24 in the Journal of Pediatrics.

Never leave children in a car seat with unbuckled or partially buckled straps, and never place car seats on soft or unstable surfaces, Batra said in a journal news release.

Also, parents need to know that straps on bouncers, strollers and swings may not prevent infants from getting into dangerous positions. Never place more than one infant in a swing meant for only one child, the researchers added.

Slings are particularly dangerous because of their design. When using a sling, the infant’s face should be “visible and kissable” at all times, the study authors said.

Sleep-related deaths are the most common cause of death among American infants aged 1 month to 12 months. Infants should sleep on their back on a firm mattress, without loose bedding, the American Academy of Pediatrics says.