Even when parents know they're being watched, they still put babies to sleep in unsafe ways that are linked to an increased risk of sudden infant death syndrome (SIDS), a U.S. study suggests.

Researchers analyzed nighttime video recordings taken in family homes when babies were 1 month, 3 months and 6 months old.

On camera, the vast majority of infants were sleeping in an unsafe position or surrounded by items that increase the risk of SIDS like pillows, loose bedding, stuffed animals and crib bumpers, the study found.

"SIDS is not that rare; about 3,500 babies die of sudden unexplained infant deaths each year in the US," said senior study author Dr. Ian Paul of Penn State College of Medicine in Hershey, Pennsylvania.

"Given that these are preventable deaths, this severe risk is notable," Paul added by email.

Nationwide, SIDS kills about four babies out of every 10,000 live births, down from about 130 in 10,000 in 1990, according to the Centers for Disease Control and Prevention.

Despite the dramatic decline in death from SIDS since 1992, when the American Academy of Pediatrics (AAP) announced that babies should be placed on their backs to sleep, SIDS remains a leading cause of infant mortality.

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To prevent SIDS, the AAP also encourages breastfeeding, pacifier use and firm crib mattresses while advising against blankets, pillows and bed-sharing.

Among the 160 one-month-olds recorded for the current study, 21 percent were put to sleep on unsafe surfaces, 14 percent weren't on their backs and 91 percent had loose items like stuffed animals and pillows, researchers report in Pediatrics.

Of the 151 babies recorded at three months of age, 10 percent were put to sleep on unsafe surfaces, 18 percent weren't on their backs and 87 percent had unsafe items like crib bumpers or blankets.

By age 6 months, 12 percent of the 147 infants still in the study were put down on unsafe surfaces, 33 percent weren't on their backs and 93 percent had unsafe items with them in bed.

When parents moved babies during the night, they were even less likely to be on their back in the next place they slept, the study also found.

Limitations of the study include its largely white group of participants with more education and income than typical U.S. parents. That may have underestimated how often babies sleep in unsafe ways, the authors note.

The results are surprising because most of the parents were college graduates, said Michael Gradisar, a researcher at Flinders University in Adelaide, Australia, who wasn't involved in the study. These parents should be well aware of public health campaigns promoting safe infant sleep.

"It may be that they hear conflicting advice from other sources (e.g., friends and family), but we really don't know unless further research asks why parents choose unsafe sleep practices for their child," Gradisar said by email.

Some parents may also introduce unsafe items like blankets because they think these things comfort infants and help crying babies get to sleep, said Helen Ball, director of the Parent-Infant Sleep Lab at Durham University in the U.K.

"If the baby wakes up in the night - which of course most do - and can't be settled back to sleep in the same position or location, then parents tend to do what works to get them back to sleep especially if a crying baby risks waking everyone else in the household," Ball, who wasn't involved in the study, said by email.

In addition to SIDS, though, parents also need to be aware that babies may suffocate or be strangled to death when they don't sleep on their backs in a safe environment, noted Dr. Michael Goodstein, a neonatologist at York Hospital WellSpan Health in York, Pennsylvania, who wasn't involved in the study.

Because the absolute risk of SIDS is low, parents may have a false sense of security, Goodstein said by email.

"At the end of the day, it isn't a problem unless it happens to you - and then it's too late and there is no turning back," Goodstein said.