Ambient air pollution concentrations and incidence of spontaneous abortion

Background and aims: Previous epidemiologic studies suggest that air pollution exposure may be a risk factor for spontaneous abortion (SAB), i.e., pregnancy loss before 20 weeks’ gestation. However, most studies identified SABs from administrative records, which likely under-ascertain losses early in pregnancy. In a prospective preconception cohort study of couples trying to conceive, we examined the association between residential ambient air pollution concentrations and SAB incidence. Methods: Eligible female participants enrolled during 2013-2019. They were aged 21-45 years, residents of the United States or Canada, and trying to conceive without fertility treatments. Participants completed online questionnaires at enrollment, every 8 weeks during preconception, and about 8 weeks and 32 weeks of pregnancy. We restricted our analysis to the 5,513 participants who conceived within 12 cycles of follow-up. We estimated two-week average residential ambient concentrations of particulate matter <2.5 µm (PM2.5), nitrogen dioxide (NO2), and ozone (O3) using country-specific spatiotemporal models. We used Cox proportional hazards regression with gestational weeks as the time scale to estimate incidence rate ratios (IRRs) and 95% confidence intervals (CI) for an interquartile range (IQR) increase in exposure, adjusting for individual- and neighborhood-level confounders. Results: 19% of participants reported a SAB during follow-up; of these, 72% were early losses (<8 weeks of gestation, median=6 weeks). Air pollution concentrations were not appreciably associated with SAB incidence. Among US participants, IRRs for a one-IQR increase in concentrations during the two weeks before conception were 1.08 (95% CI: 0.93-1.24) for PM2.5 (IQR=3.0 µg/m3), 1.02 (95% CI: 0.90-1.15) for NO2 (IQR=5.4 µg/m3), and 1.00 (95% CI: 0.86-1.17) for O3 (IQR=10.7 µg/m3). Associations were similar among Canadian participants, and among those reporting early losses. Conclusions: Air pollution concentrations showed little association with SAB incidence in this cohort of pregnancy planners. Keywords: air pollution, preconception cohort, pregnancy loss, spontaneous abortion