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This is the VOA Special English Health Report.

Scientists say they have strong evidence that a biological problem could explain why

some babies die suddenly in their sleep.

Researchers from Children's Hospital Boston and Harvard Medical School examined brain tissue from forty-one babies. Thirty-one of them had died of sudden infant death syndrome, or SIDS; ten died of other causes.

The study appeared last week in the Journal of the American Medical Association. It adds to the belief that lying face down may increase the risk of SIDS as a result of pre-existing problems.

The researchers say they found abnormalities in nerve cells in the brainstems of the SIDS babies. The brainstem is involved in controlling breathing and waking from sleep. This part of the brain also controls blood pressure and body heat.

The researchers say the brainstems of the SIDS babies had far more of one kind of nerve cell than the other babies. These cells produce and release serotonin, a brain chemical believed to play an important part in controlling sleep.

Hannah Kinney of Children's Hospital Boston was a leader of the study. Doctor Kinney says normal babies will wake up, turn their head and start to breathe faster if they are not getting enough air. But if the serotonin system is bad, then a baby's brain might not get the message to react.

The scientists say that things like alcohol use and smoking when a woman is pregnant may harm the development of the brainstem.

They also found biological differences that they say may explain why SIDS happens two times as often in boys than girls.

They say the findings of their study could lead to a test for SIDS risk and possibly treatments someday. But they also note that the small size of their study represents a possible limitation of the research.

The United States records about two thousand cases of sudden infant death syndrome each year. SIDS is the nation's leading cause of death for babies between one month and twelve months of age.

But the number of cases has fallen in the past few years. This follows the launch of a campaign to urge people to place babies under one year on their back to sleep. Still, sixty-five percent of the SIDS babies in this study were found on their stomach or side.

And that's the VOA Special English Health Report, written by Caty Weaver. For more health news, with transcripts and MP3 files, go to www.unsv.com. I'm Steve Ember.