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

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 1st January 2000 and 31st December 2015 in Western Australia were linked to fine spatiotemporal monthly PM_2.5 concentrations. Monthly distributed lag linear and non-linear (DLM and DLNM) 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/m³. Pregnancy PM_2.5 exposure showed small dose-response associations with stillbirth and sPTB with critical susceptible periods spanning the 3rd–6th gestational months, especially from the DLNM method. Relative to 5 μg/m³ from the DLM method, 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 4th 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/m³ 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.