Reflected in the mirror of a crib, a child care worker watches over an infant in New York in 2007. (Kathy Willens/Associated Press)

More than half of sudden infant deaths in a British review published Wednesday occurred while the baby shared a bed or sofa with a parent who often had been drinking, taking drugs or smoked.

The incidence of sudden infant death syndrome, or SIDS — the sudden and unexpected death of an apparently healthy infant under one year of age — has gone down by almost 50 per cent over the past decade.

In that time, public awareness campaigns in developed countries have been encouraging parents to:

• Put infants to sleep on their back.
• Avoid placing pillows and other soft materials in the crib.
• Quit smoking.

But researchers writing in Wednesday's issue of the British Medical Journal suggest that parents also need to be advised to avoid dangerous co-sleeping arrangements.

Peter Fleming of St. Michael's Hospital in Bristol, England, and his colleagues studied 80 unexplained SIDS cases that occurred in southwestern England from 2003 through 2006.

The team compared the SIDS cases to two groups of infants, one of 82 "high-risk" infants of smoking, socially deprived, single mothers with two or more kids, and the other with 87 babies from randomly selected families.

Co-sleeping, alcohol and drug use

Of the SIDS infants, 54 per cent died while sleeping with a parent. And the researchers found that drinking and drug use also played a role.

"Much of this excess may be explained by a significant … interaction between co-sleeping and recent parental use of alcohol or drugs," the researchers said.

After the three risk factors were taken into account, there was no added risk to taking a baby to bed, Fleming noted.

But being a smoker alone impairs responsiveness when sleeping, he said.

In the two control groups, the rate of co-sleeping was about 20 per cent.

The researchers also found that one-fourth of the infants who died were swaddled, and one-fifth used a pillow.

The study's authors concluded that some parents may have been confused. They suggested the stringent message against sharing a bed with the child may have led them to believe the sofa is safer.

Yet the findings suggest that sharing a sofa is more dangerous than sharing a bed, but the researchers don't know why. Fleming speculated that overheating may be a factor.

The researchers suggested parents may need to be given more specific warnings about risk factors for SIDS, such as:

• Falling asleep with a baby on the couch.
• Sleeping with a baby when the caregiver is drunk or on drugs.

Meanwhile, parents continue to debate whether bed-sharing facilitates feedings, while others fear rolling onto the child in the middle of the night.

The Canadian Pediatric Society published a set of recommendations for safe sleeping environments five years ago that suggested the safest place for a baby to sleep is on its back in a standard crib.

Despite the risks, the society noted many families choose to sleep together.