The federal "Back to Sleep" campaign, which was launched in 1992, has been credited with cutting in half the number of babies who have died from sudden infant death syndrome. Babies placed on their backs are thought to be much less likely to stop breathing during sleep, often the cause of death in SIDS, than babies placed on their stomachs. But recent drops in the death rate may be not be because of fewer babies dying of SIDS, say researchers from the University of Texas Medical Branch. Instead, decreases may stem from physicians or medical examiners who classify the death differently, making SIDS more common than researchers have realized.

What the researchers wanted to know: How much of a reduction in infant deaths from SIDS has there been since 1992?

What they did: The researchers looked at several different categories of U.S. mortality data in children between the ages of one month and one year from 1950 to 2001. They concentrated on deaths in babies since 1992 because that was when the Back to Sleep campaign began. They compared rates of deaths attributed to SIDS to deaths from all causes and from other causes that could have symptoms similar to SIDS, such as suffocation, aspiration, homicide, or unknown death.

What they found: The overall infant mortality rate declined 55 percent between 1992 and 2001, from 314 deaths per 100,000 births to 231 deaths per 100,000 births. The rate of SIDS deaths declined in that same period at an average of 8.6 percent per year. Looking at the end of that time period, however, from 1999 to 2001, infant mortality leveled off, though there was still an apparent drop in SIDS deaths. Also, during that same two-year period, deaths from an unknown cause rose. The researchers say that some of the deaths between 1999 and 2001 may have been misclassified so that the apparent drop in SIDS deaths is not what's actually happening.

What it means to you: After declining for years, the death rate from SIDS may have leveled off. If deaths that once would have been attributed to SIDS are no longer being classified that way on death reports, then there is no way to tell which way the trend is actually going. If the trend truly is leveling off, it may be because some parents are still put infants to sleep on their stomachs or in dangerous environments. On the other hand, the leveling off could be seen as further support for recent research that suggests genetic mutations may predispose babies to sudden infant death and that a certain number of infants will die regardless of how their parents put them to bed.

Caveats: The classification system for all deaths changed between 1998 and 1999. Though the researchers tested to make sure they could compare data from before and after the change, there still might have been differences in the two systems that make it hard to compare data from before 1999 and after that year.

Find out more: The SIDS Alliance has information for expectant and new parents.

One mother started www.battlesids.org after she lost her daughter to SIDS.

The National Institute of Child Health and Human Development has information on the Back to Sleep campaign.

Read the article: Malloy, M.H. and MacDorman, M. "Changes in the Classification of Sudden Unexpected Infant Deaths: United States, 1992-2001." Pediatrics. May 2005, Vol. 115, No. 5, pp. 1247-1253.