The analysis, published today in Pediatrics, found that in 2002 preterm birth, birth at less than 37 completed weeks gestation, contributed to more than one-third of infant deaths within the first year of life.
"We have long known that babies born too soon face many challenges ?even death," said Joann Petrini, Ph.D., director of the March of Dimes Perinatal Data Center. "But this research confirms the urgent role preventing preterm birth can play in improving infant mortality in the United States."
The U.S. infant mortality rate has declined since 1995 except between 2001 and 2002, when the rate increased for the first time since 1958.
In 2002, the National Center for Health Statistics listed short gestation/low birth weight as the cause of 17 percent of infant deaths. However, two-thirds of the infants who died in 2002 were born prematurely. This difference led the CDC researchers to speculate that premature birth plays a greater role in infant death than the official reporting system indicates.
The researchers proposed a new method to determine the contribution of premature birth to infant mortality. They reviewed causes of infant death and combined conditions, such as respiratory distress syndrome, which frequently occur in premature babies. This new method doubled the percentage of infant deaths caused by premature birth to 34.3 percent.
More than 95 percent of those deaths were of infants who were born at less than 32 weeks gestation, the definition of infants who are "very preterm."
Birth defects are the leading cause of infant death, followed by prematurity, according to the official reporting systems. But, using this new classification, premature birth would be the most frequent cause of infant death.
More than a half million babies are born too soon each year and the preterm birth rate has increased more than 30 percent since 1981. Babies who do survive face risks of lifelong challenges of cerebral palsy, mental retardation, chronic lung disease, and vision and hearing loss, as well as other developmental problems.