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.