SEATTLE — Preterm birth is the most frequent cause of infant mortality in the United States, accounting for one-third of infant deaths in 2002, according to data presented at a meeting of the Society for Pediatric and Perinatal Epidemiologic Research.
Prematurity remains a major cause of infant morbidity and mortality in the United States, and rates continue to rise. Across the board, the rate of premature births rose 13% between 1992 and 2002, with seven states showing increases of 30% or higher, according to the March of Dimes.
“As of yet we do not have good predictors and prevention for preterm birth,” Dr. William Callaghan said in an interview.
“We decided to reassess the contribution of preterm birth to infant mortality, in light of the strong connection between prematurity and infant death and the rising rates of preterm birth,” explained Dr. Callaghan, a senior scientist in the Maternal and Infant Health Branch, Division of Reproductive Health, at the CDC.
Although about two-thirds of infant deaths occur in those born at less than 37 weeks' gestation, fewer than 20% of infant deaths are classified as being due to preterm birth using the standard National Center for Health Statistics' classification of “leading cause of death.”
This seeming contradiction can be explained, Dr. Callaghan said. “Among all infants who died, 65% were born preterm. However, just because the association exists does not mean that being preterm was necessarily the cause of death.”
As an example, if a preterm infant with a lethal congenital anomaly dies, one cannot say with any certainty that the death was caused by preterm birth or by the congenital anomaly.
Dr. Callaghan explained that he and his coauthors took a very conservative strategy and looked only at the 20 leading causes of infant death. Of 27,970 records in the linked birth/infant death file for 2002, the 20 leading causes accounted for 22,273 or 80% of all infant deaths. Within this group, they assessed the contributing role of preterm birth for each of the causes of death that are embedded in the standard categories.
The methodology was broken down into three basic steps to determine whether the cause of death was actually related to prematurity. First, the cause of death had to be statistically associated with preterm birth. If preterm infants contributed to at least 75% of the mortality due to a specific cause, the death was considered as potentially due to preterm birth.
Second, information about the condition had to be sufficient in order to determine if preterm birth actually led to that condition, or if the condition designated as the cause of death led to preterm birth.
Finally, it had to be determined whether the infant who died from the proposed cause was actually preterm.
Not surprisingly, the earliest infants had the highest rate of mortality, said Dr. Callaghan. Infants born at less than 32 weeks' gestation and who weighed less than 1,500 g accounted for 88% of the total 9,596 deaths attributed to preterm birth.
“If you look at all infant deaths in total, preterm births account for 34% of them,” Dr. Callaghan said. “But if you look at infant deaths confined to the 20 leading causes, then that rate is 43%.”
The majority of the infants die within the first week of life, with two-thirds of these deaths occurring during the first 24 hours after birth. “Prevention of preterm birth, especially at the earliest gestations, is crucial if we are going to further reduce the infant mortality rate,” he said.