Premature births account for more than a third of infant deaths in the U.S, nearly twice previous estimates, researchers say.

In 2002, official U.S. health statistics listed low birth weight or short gestation — less than 37 weeks — as the cause of 17 per cent of infant deaths. But that number could be low, based on a re-analysis of the data.

Some causes of newborn death are related to premature birth but haven't been counted as deaths from premature birth and should be, the researchers said. The preterm-related conditions include:

  • Respiratory distress syndrome caused by underdeveloped lungs.
  • Brain hemorrhage.
  • Maternal complications such as premature rupture of membranes.

"The only way that an infant gets assigned [preterm  birth] is if there's nothing else on the death certificate," said Dr. Bill Callaghan, lead author of the study in Monday's issue of the journal Pediatrics.

"That may result in an underestimation of what the real problem is," said Callaghan, a senior scientist at the U.S. Centers for Disease Control's maternal and infant health branch.

Including the deaths from related causes may strengthen calls for pregnant women to avoid smoking and drug use.

Groups that lobby for more research into unknown causes of preterm births, such as the March of Dimes, could also use the new statistics to boost their case for increased funding.

Reducing infant mortality

"This study found that efforts to reduce infant mortality rates must focus on preterm birth," and find ways to safely delay such births," the researchers wrote.

To reclassify the statistics, Callaghan and his colleagues analyzed about 28,000 infants deaths in 2002.

About 4,600 or 17 per cent were attributed to preterm birth alone, but adding in the preterm-related conditions boosted the number to 9,600 births, or 34 per cent.

The researchers listed several causes for the rising preterm birth rate in the U.S., including an increasing number of births to teenaged girls and older women, and more frequent use of in vitro fertilization which tends to produce more multiple births and earlier births.

Infant mortality rates in the U.S. declined during most of the last century but have stabilized recently as preterm births have risen.

With files from the Associated Press