Prematurity Is Root Cause of a Third of U.S. Infant Deaths

ATLANTA, Oct. 2 -- Preterm birth is the leading cause of infant mortality in the U.S., according to CDC researchers, accounting for at least a third of all babies' deaths in 2002.

The contribution of prematurity to infant mortality may be twice as high as originally estimated, reported William M. Callaghan, M.D., M.P.H., and CDC colleagues, in the October issue of Pediatrics.

They looked at the top 20 causes of infant deaths in the U.S. in 2002, and found that 34% of the deaths occurred in preterm infants, 95% of whom were born before 32 weeks gestational age and weighed less than 1,500 g (3.3 lb).

Two-thirds of the deaths in preterm infants occurred in the first 24 hours of life, the investigators found.

"Efforts to prevent infant deaths attributable to preterm birth require safely delaying birth until a later gestational age, when survival is more likely," the authors wrote. "Therefore, there is an urgent need for an expanded comprehensive agenda to understand the complex social and biological factors that determine susceptibility to preterm birth, to detect women at risk early in pregnancy, and to develop and to evaluate new methods for preventing this important cause of infant death."

Standard methods of estimating infant mortality, using International Classification of Diseases, 10th Revision (ICD-10) codes, have yielded an estimate of 17% by this method, the National Center for Health Statistics calculated.

But while this type of classification allows monitoring of trends over time, it "does not capture adequately the overall contribution of preterm birth (less than 37 weeks of gestation) to the national infant mortality rate, because the relationship between preterm birth and death during the first one year of life is not distinctly identifiable by using available cause-of-death titles," the investigators wrote.

Instead, the CDC researchers developed an approach in which deaths due to conditions that cause premature birth, or result from it, are considered to be the cause of death based on biological factors.

The investigators identified the top 20 leading causes of infant (less than one year of age) death in 2002 using the U.S. linked birth/infant death data set. They assessed the contribution of preterm birth to each cause by determining the proportion of infants who were born preterm for each cause of death, and by considering the biological connection between preterm birth and the specific cause of death.

For example, the category "disorders related to short gestation and low birth weight, not elsewhere classified," which accounted for 4,636 infants deaths in the United States in 2002, was the second largest cause of death overall, and the leading cause of death among preterm children. Perhaps not surprisingly, 93.6% of all children in this category were preterm.

Of the 27,970 recorded deaths in the linked file for 2002, the 20 leading causes accounted for 22,273 deaths, or 80% of all infant deaths. Deaths of preterm infants in the top 20 categories totaled 9,596 (34.3%).

The top five causes of death overall listed with their associated ICD-10 category, were:

• Congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99), occurring in 5,630 infants, 49.5% of whom were preterm.
• Disorders related to short gestation and low birth weight, not elsewhere classified (P07), occurring in 4,636 infants, 93.6% preterm.
• Sudden infant death syndrome (R95), 2,295 infants, 23.2% preterm.
• Newborn affected by maternal complications of pregnancy (P01) 1,704 children, 91.3% preterm.
• Newborn affected by complications of placenta, cord, and membranes (P02), 1,013 deaths, 87.5% in preterm infants.

Among preterm infants, short gestation/low birth was the leading cause, followed by maternal complications of pregnancy (incompetent cervix, premature membrane rupture, multiple pregnancy); complications of placenta, cord and membranes (e.g., placenta previa); respiratory distress, and bacterial sepsis.

More than 66% of deaths attributable to preterm birth occurred during the first 24 hours of life, and only 7% occurred after the first four weeks. Deaths most likely to occur within 24 hours of birth were those attributable to short gestation/low birth weight, atelectasis, maternal complications, and cord and placental complications.

"On the basis of this evaluation, preterm birth is the most frequent cause of infant death in the United States, accounting for at least one third of infant deaths in 2002," the authors wrote. "The extreme prematurity of most of the infants and their short survival indicate that reducing infant mortality rates requires a comprehensive agenda to identify, to test, and to implement effective strategies for the prevention of preterm birth."