The contribution of mild and moderate preterm birth to infant mortality

Context The World Health Organization defines preterm birth as birth at les s than 37 completed gestational weeks, but most studies have focused on ver y preterm infants (birth at <32 weeks) because of their high risk of mortal ity and serious morbidity. However, infants born at 32 through 36 weeks are more common and their public health impact has not been well studied. Objective To assess the quantitative contribution of mild (birth at 34-36 g estational weeks) and moderate (birth at 32-33 gestational weeks) preterm b irth to infant mortality. Design, Setting, and Participants Population-based cohort study using linke d singleton live birth-infant death cohort files for US birth cohorts for 1 985 and 1995 and Canadian birth cohorts (excluding Ontario) for 1985-1987 a nd 1992-1994. Main Outcome Measures Relative risks (RRs) and etiologic fractions (EFs) fo r overall and cause-specific early neonatal (age 0-6 days), late neonatal ( age 7-27 days), postneonatal (age 28-364 days), and total infant death amon g mild and moderate preterm births vs term births (at greater than or equal to 37 gestational weeks). Results Relative risks for infant death from ail causes among singletons bo rn at 32 through 33 gestational weeks were 6.6 (95% confidence interval [CI ], 6.1-7.0) in the United States in 1995 and 15.2 (95% CI, 13.2-17.5) in Ca nada in 1992-1994; among singletons born at 34 through 36 gestational weeks , the RRs were 2.9 (95% CI, 2.8-3.0) and 4.5 (95% CI, 4.0-5.0), respectivel y. Corresponding EFs were 3.2% and 4.8%, respectively, at 32 through 33 ges tational weeks and 6.3% and 8.0%, respectively, at 34 through 36 gestationa l weeks; the sum of the EFs for births at 32 through 33 and 34 through 36 g estational weeks exceeded those for births at 28 through 31 gestational wee ks. Substantial RRs were observed overall for the neonatal (eg, for early n eonatal deaths, 14.6 and 33.0 for US and Canadian infants, respectively, bo rn at 32-33 gestational weeks; EFs, 3.6% and and 6.2% for US and Canadian i nfants, respectively) and postneonatal (RRs, 2.1-3.8 and 3.0-7.0 for US and Canadian infants, respectively, born at 32-36 gestational weeks; EFs, 2.7% -5.8% and 3.0%-7.0% for the same groups, respectively) periods and for deat h due to asphyxia, infection, sudden infant death syndrome, and external ca uses. Except for a reduction in the RR and EF for neonatal mortality due to infection, the patterns have changed little since 1985 in either country. Conclusions Mild- and moderate-preterm birth infants are at high RR for dea th during infancy and are responsible for an important fraction of infant d eaths.