Interpregnancy intervals and the risk for infant mortality: a case control study of Arizona infants 2003-2007

There is well-documented evidence on how interpregnancy interval (IPI) is associated with adverse perinatal outcomes and how short and long IPIs are associated with increased risk for preterm birth, low birth weight, and intra-uterine growth restriction. However, the extremes of IPI on infant mortality are less well documented. The current study builds on the existing evidence on IPI to examine if extremes of IPI are associated with infant mortality, and also examines if IPI is associated with both neonatal and post-neonatal mortality after adjusting for several known confounders. Matched birth and death certificate data for Arizona resident infants was drawn for 2003-2007 cohorts. The analysis was restricted to singleton births among resident mothers with a previous live birth (n = 1,466) and a randomly selected cohort of surviving infants during the same time-frame was used as a comparison group (n = 2,000). Logistic regression models were utilized to assess the odds for infant mortality at monthly interpregnancy intervals (<6, 6-11, 12-17, 18-23, 24-59, ≥60), while adjusting for established predictors of infant mortality (i.e., preterm birth, low birth weight, and small for gestational age), and other potential confounders. Unadjusted analysis showed greater clustering at extreme IPIs of <6 months and ≥60 months for infants that died (32%) compared to infants that survived (24.7%). Shorter IPI (i.e., <6 months, 6-11 months, and 12-17 months) compared to 'ideal' IPI (i.e., 18-23 months), were associated with infant mortality even after adjusting for confounders. Short intervals were significantly associated with neonatal, but not post-neonatal deaths. IPI above 23 months were not associated with infant mortality in our analyses. Shorter IPIs (18 months or less) significantly increases the risk for neonatal infant mortality even after controlling for known confounders, and our study adds to the existing evidence on adverse perinatal outcomes. Counseling women of reproductive age on the benefits of spacing pregnancies to at least 18 months addresses one preventable risk for early infant mortality.