Critical periods of maternal exposure to ambient fine particulate matter and the risks of stillbirth and spontaneous preterm birth in Western Australia

• Low-level monthly PM2.5 exposure associated with birth outcomes in Western Australia • The most sensitive windows for stillbirth were 5th–6th gestational months • The most sensitive windows for spontaneous PTB were 3rd–5th gestational months • The PM2.5 effect on birth outcomes showed sociodemographic disparities • There was PM2.5 –biothermal stress synergistic interaction effect on the birth outcomes Few studies have investigated weekly or monthly exposure-lag-response association between fine particulate matter (PM 2.5 ) and preterm birth, and there has been no known such study for stillbirth. We aimed to identify potential critical susceptible periods of the association between monthly PM 2.5 exposure during pregnancy and stillbirth and spontaneous preterm birth (sPTB). A total of 414,771 singleton births, of which 0.5% and 3.9% were stillbirth and sPTB respectively, between 1 st January 2000 and 31 st December 2015 in Western Australia were linked to fine spatiotemporal monthly PM 2.5 concentrations. Monthly distributed lag linear and non-linear Cox regressions were performed to investigate PM 2.5 exposure during pregnancy and covariate-adjusted hazards of stillbirth and sPTB. The mean PM 2.5 exposure during pregnancy was 8.1 μg/m 3 . Pregnancy PM 2.5 exposure showed small dose-response associations with stillbirth and sPTB with critical susceptible periods spanning the 3 rd –6 th gestational months. Relative to 5 μg/m 3 , the strongest hazards of PM 2.5 exposure were 1.04 (95% CI 0.99, 1.08) and 1.03 (95% CI 1.02, 1.05) during the 4 th gestational month for stillbirth and sPTB, respectively. Monthly exposures at late pregnancy were associated with lower hazards. The average pregnancy exposure hazards were 1.08 (95% CI 0.88, 1.32) and 1.07 (95% CI 0.99, 1.15) per 5 μg/m 3 increment for stillbirth and sPTB, respectively. The average entire pregnancy exposure showed potential attributable burden, synergistic interaction effects with biothermal stress, and comparatively higher hazards for female births and births to mothers who were Caucasian, < 35 years old, high SES, nulliparous, and smoked during pregnancy.