In latest analysis, premature births seen as top cause of infant mortality

The usual statistics show that the single biggest cause of infant death in the United States is birth defects, such as spina bifida and heart malformations.

But today, federal researchers report that, according to a new method of analysis, the most common killer of American babies is actually premature birth.

Prematurity ``is the most frequent reason that infants die, no question," said Dr. William M. Callaghan, lead author of the paper that appears in this month's issue of the journal Pediatrics.

``If we're going to make any inroads into infant mortality, we're going to have to figure out how to prevent babies from being born too early and too small," said Callaghan, senior scientist in the Maternal and Infant Health Branch at the Centers for Disease Control and Prevention.

Of course, that's not easy: It's unclear exactly what causes most prematurity, though multiple births, smoking, and drug abuse contribute in some cases.

Birth defects directly accounted for only about 20 percent of infant deaths, Callaghan's examination of nearly 28,000 reported infant deaths in 2002 showed, while prematurity accounted for at least 34 percent. Even that is a conservative estimate, he said. Other causes of infant deaths include accidents, pneumonia, and sudden infant death syndrome.

Premature births have been on the rise in the last 25 years, and now account for about 12 percent of live births, or about a half-million American babies a year.

The great majority survive and thrive, but prematurity is believed to be partly to blame for the country's failure to keep lowering its infant mortality rates in recent years.

Callaghan's paper ``puts hard numbers on the fact that prematurity is a bigger problem than people think," said Joann Petrini, director of the March of Dimes Perinatal Data Center.

CAREY GOLDBERG