Beyond birth outcomes: Interpregnancy interval and injury-related infant mortality

Background Several studies have examined the association between IPI and birth outcomes, but few have explored the association between interpregnancy interval (IPI) and postnatal outcomes. Objective We examined the association between IPI and injury-related infant mortality, a leading cause of postneonatal mortality. Methods We used 2011-2015 US period-linked birth-infant death vital statistics data to generate a multiyear birth cohort of non-first-born singleton births (N = 9 782 029). IPI was defined as the number of months between a live birth and the start of the pregnancy leading to the next live birth. Causes of death in the first year of life were identified using ICD-10 codes. Hazard ratios (HR) for IPI categories were estimated using Cox proportional hazards models adjusted for birth order, county poverty level, and maternal characteristics (marital status, race/ethnicity, education, age at previous birth). Results After adjustment, overall infant mortality (48.1 per 10 000 births) was higher for short and long IPIs compared with IPI 18-23 months (reference): <6, aHR 1.61, 95\% CI 1.54, 1.68; 6-11, aHR 1.22, 95\% CI 1.17, 1.26; and 60+ months, aHR 1.12, 95\% CI 1.08, 1.16. In comparison, the risk of injury-related infant mortality (4.4 per 10 000 births) decreased with longer IPIs: <6, aHR 1.77, 95\% CI 1.55, 2.01; 6-11, aHR 1.41, 95\% CI 1.25, 1.59; 12-17, aHR 1.25, 95\% CI 1.10, 1.41; 24-59, aHR 0.78, 95\% CI 0.69, 0.87; and 60+ months, aHR 0.55, 95\% CI 0.48, 0.62. Conclusion Unlike overall infant mortality, injury-related infant mortality decreased with IPI length. While injury-related deaths are rare, these patterns suggest that the timing between births may be a marker of risk for fatal infant injuries. The first year postpartum may be an ideal time for the delivery of evidence-based injury prevention programmes as well as family planning services.