Rates of sudden infant death from suffocation or strangulation have quadrupled in the past 20 years in the United States, most apparently from parents sleeping with their babies, government researchers reported on Monday.

Black male babies are the most affected but it is not clear why, the researchers at the U.S. Centers for Disease Control and Prevention reported.

The trend is clear despite successful campaigns to prevent sudden infant death syndrome by putting babies to sleep on their backs instead of their tummies, the CDC reported in the journal Pediatrics.

"Infant mortality rates attributable to accidental suffocation and strangulation in bed have quadrupled since 1984," the CDC's Carrie Shapiro-Mendoza and colleagues wrote.

"Prevention efforts should target those at highest risk and focus on helping parents and caregivers provide safer sleep environments."

Evidence shows that babies should be laid to sleep alone, on a flat mattress, with no loose pillows or blankets and in a crib with bars designed to prevent entrapment.

Shapiro-Mendoza and colleagues went through national death statistics from 1984 to 2004.

"Sudden, unexpected infant death was defined as a combination of all deaths attributed to accidental suffocation and strangulation in bed, sudden infant death syndrome, and unknown causes," they wrote.

Rates of sudden infant death syndrome, also known as cot death or crib death, have plummeted in countries where health workers have counseled parents and caregivers to put infants on their backs to sleep, to avoid keeping rooms too warm and to keep loose blankets and pillows away from infants.

The data reflected this with sudden, unexpected infant deaths overall falling from 160 per 100,000 in 1984, or 5,885 total, to 92.4 in 2004 or 3,798 total.

Rates of strangulation or suffocation, however, rose by 14 percent between 1996 and 2004.

Most of the deaths that could be determined were by "overlay" — the parent rolling over onto the child.

Other causes include suffocating in soft bedding, becoming wedged between a mattress and frame or wall, or getting a head caught in something.

"To our knowledge, it is the first study to document the national trend showing a fourfold increase in accidental suffocation and strangulation in bed infant mortality in recent years," the CDC team wrote.