Out-of-hospital births and infant mortality in the United States: Effect measure modification by rural maternal residence

Background Out-of-hospital births have been increasing in the United States, and home births are almost twice as common in rural vs. urban counties. Planned home births and births in rural areas have each been associated with an increased risk of infant mortality. Objectives To estimate the effect of birth setting on infant mortality in the United States and how this is modified by rural-urban county of maternal residence. Methods We conducted a population-based cohort study of infants born in the United States during 2010-2017 using the National Center for Health Statistics' period-linked birth-infant death files. Unadjusted and adjusted Poisson regression models were used to calculate infant mortality rate ratios and 95% confidence intervals for out-of-hospital births vs. hospital births stratified by maternal residence. Relative excess risk due to interaction (RERI) was calculated to assess effect measure modification on the additive scale. Results The study included 25,210,263 live births. Of rural births, 97.8% was in hospitals, 0.5% was in birth centres, and 1.5% was planned home births; of urban births, 98.6% was in hospitals, 0.5% was in birth centres, and 0.7% was planned home births. After adjusting for maternal demographics and markers of high-risk pregnancy and stratifying by maternal residence, infant mortality rates were generally higher for out-of-hospital as compared to hospital births (e.g. rural planned home births aRR 1.62, 95% confidence interval [CI] 1.42, 1.85, and rural birth centre aRR 1.33, 95% CI 1.05, 1.68). There were positive additive effects of rural residence on infant mortality for planned home births and birth centre births. Conclusions Within both rural and urban areas, out-of-hospital births generally had higher rates of infant mortality than hospital births after accounting for maternal demographics and markers of high-risk pregnancy. The risks associated with planned home births and birth centre births were more pronounced for women in rural counties.