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Michael S. Kramer, MD; Kitaw Demissie, MD, PhD; Hong Yang, MSc; Robert W. Platt, PhD; Reg Sauvé, MD, MPH; Robert Liston, MD; for the Fetal and Infant Health Study Group of the Canadian Perinatal Surveillance System

JAMA. 2000;284:843-849.

Context  The World Health Organization defines preterm birth as birth at less than 37 completed gestational weeks, but most studies have focused on very preterm infants (birth at <32 weeks) because of their high risk of mortality 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 gestational weeks) and moderate (birth at 32-33 gestational weeks) preterm birth to infant mortality.

Design, Setting, and Participants  Population-based cohort study using linked singleton live birth–infant death cohort files for US birth cohorts for 1985 and 1995 and Canadian birth cohorts (excluding Ontario) for 1985-1987 and 1992-1994.

Main Outcome Measures  Relative risks (RRs) and etiologic fractions (EFs) for 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 among mild and moderate preterm births vs term births (at 37 gestational weeks).

Results  Relative risks for infant death from all causes among singletons born 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 Canada 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), respectively. Corresponding EFs were 3.2% and 4.8%, respectively, at 32 through 33 gestational weeks and 6.3% and 8.0%, respectively, at 34 through 36 gestational weeks; the sum of the EFs for births at 32 through 33 and 34 through 36 gestational weeks exceeded those for births at 28 through 31 gestational weeks. Substantial RRs were observed overall for the neonatal (eg, for early neonatal deaths, 14.6 and 33.0 for US and Canadian infants, respectively, born at 32-33 gestational weeks; EFs, 3.6% and and 6.2% for US and Canadian infants, 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 death due to asphyxia, infection, sudden infant death syndrome, and external causes. 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 death during infancy and are responsible for an important fraction of infant deaths.

Author Affiliations: Departments of Pediatrics and of Epidemiology and Biostatistics, McGill University, Faculty of Medicine, Montreal, Quebec (Drs Kramer and Platt and Mr Yang); Bureau of Reproductive and Child Health, Laboratory Centre for Disease Control, Health Canada, Ottawa, Ontario (Dr Demissie); Department of Environmental and Community Medicine, University of Medicine and Dentistry of New Jersey, Piscataway (Dr Demissie); Departments of Pediatrics and Community Medicine, University of Calgary Faculty of Medicine, Calgary, Alberta (Dr Sauvé); and Department of Obstetrics and Gynecology, University of British Columbia, Vancouver (Dr Liston).

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August 16, 2000
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