Abstract
Air pollution has been examined in relation to stillbirth in few studies with inconsistent findings. No prior studies have examined the interaction of maternal asthma, a common pregnancy complication, and air pollution in relation to stillbirth risk. Our retrospective cohort analysis of 223,375 singleton deliveries, ≥23 weeks of gestation, from 12 clinical sites across the United States utilized electronic medical records. Average exposure to criteria air pollutants was calculated by modified Community Multiscale Air Quality models for the week prior to delivery, whole pregnancy and the first trimester. Poisson regression with generalized estimating equations estimated the relative risk (RR) of stillbirth and 95% confidence intervals in relation to an interquartile range (IQR) increase in pollutant after adjustment for age, race, parity, smoking, alcohol, pre-pregnancy body mass index, insurance, marital status, hypertension, diabetes, season of conception, birth year and study site. Maternal asthma was included with an interaction term to assess potential risk differences by asthma status. We observed 88 stillbirths among asthmatics (n=16,948, 0.52%) and 904 among non-asthmatics (n=205,435, 0.44%; p=0.14). Asthma significantly modifies the stillbirth risk (interaction p<0.05) associated with an IQR increase in whole pregnancy particulate matter <2.5 µm (PM2.5; RR=1.16 vs. 1.10), and first trimester average exposure to PM2.5 (RR=1.07 vs. 0.98), sulfur dioxide (RR=0.97 vs. 0.87), nitrogen oxides (RR=0.97 vs. 0.90) and carbon monoxide (CO; RR=1.09 vs. 0.92). Effect modification by asthma was not significant for ozone and PM <10 µm, although ozone had a main effect on stillbirth. Acute exposure risk in the week prior to delivery also did not differ by asthma status. Despite low power due to a small number of cases, our findings suggest that PM2.5 and CO exposure could differentially increase stillbirth risk for asthmatic women and merit further investigation.
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