Chronic and acute air pollution has been studied in relation to stillbirth with inconsistent findings. We examined stillbirth risk in a retrospective cohort of 223,375 singleton deliveries from 12 clinical sites across the United States. Average criteria air pollutant exposure was calculated using modified Community Multiscale Air Quality models for the day of delivery and each of the seven days prior, whole pregnancy, and first trimester. Poisson regression models using generalized estimating equations estimated the relative risk (RR) of stillbirth and 95% confidence intervals (CI) in relation to an interquartile range increase in pollutant with adjustment for temperature, clinical, and demographic factors. Ozone (O₃) was associated with a 13-22% increased risk of stillbirth on days 2, 3, and 5-7 prior to delivery in single pollutant models, and these findings persisted in multi-pollutant models for days 5 (RR = 1.22, CI = 1.07-1.38) and 6 (RR = 1.18, CI = 1.04-1.33). Whole pregnancy and first trimester O₃ increased risk 18-39% in single pollutant models. Maternal asthma increased stillbirth risk associated with chronic PM2.5 and carbon monoxide exposures. Both chronic and acute O₃ exposure consistently increased stillbirth risk, while the role of other pollutants varied. Approximately 8000 stillbirths per year in the US may be attributable to O₃ exposure.