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Background: Although mounting evidence have linked traffic-related air pollution (TRAP) with increased risk of preterm birth (PTB), whether it can interact with indoor environmental factors remains unknown, and its window(s) susceptibility at the stage of gestation is unclear. Objective: To explore PTB risk for prenatal exposure to traffic-related air pollution and home environmental factors during pregnancy, so as to identify critical window(s) in the combined effect of traffic air pollution and main home environmental factor(s) on PTB development. Methods: A retrospective cohort study of 3,509 preschool children was performed in Changsha, China during 2011-2012. The PTB prevalence was reported by the parents based on a questionnaire. We estimated each mother's exposure to traffic-related air pollutant NO2 in different windows of gestation, including conception month, three trimesters, birth month, and whole gestation. Maternal exposure to home environmental factors was considered by renovation (new furniture/redecoration) in pregnancy, and mold/damp stains and window condensation during perinatal period. Associations of PTB with both ambient NO2 and home environmental factors, and their interactions on PTB were evaluated by logistic regression models using odds ratio (OR) with 95% confidence interval (CI). Results: Traffic air pollutant NO2 exposure in utero was significantly associated with PTB, with adjusted odds ratio (OR) (95% CI) of 1.41 (1.00-1.98) for an IQR increase in NO2 exposure during whole pregnancy, particularly in the conception month and 1st trimester. We further found a positive relationship between perinatal exposure to mold/damp stains in the homes and PTB, OR (95% CI) = 1.73 (1.04-2.90). Especially, we detected a significant interaction between outdoor NO2 and indoor mold/damp stains on PTB risk. Male and female foetus were respectively more susceptible to perinatal mold/dampness at home and outdoor NO2 exposure in early gestation. Conclusion