Background: Adverse birth outcomes are more frequent among mothers with inflammatory bowel diseases (IBDs) than non-IBD mothers. In recent studies, air pollution, such as high concentrations of nitrogen dioxide (NO2), is reckoned as a risk factor for preterm birth in the general population. In this study, we investigated whether IBD mothers are at higher risk of preterm birth when exposed to NO2 compared to non-IBD mothers.Methods: We used information from the Norwegian Mother, Father and Child Cohort Study (MoBa). The pregnancy cohort was linked to the Norwegian Medical Birth Registry and air-pollution exposure data available from a subset of the study cohort. The relevant outcome in this study was preterm birth. A total of 16,170 non-IBD and 92 IBD mothers were included in the study.Results: The mean exposure of NO2 during the pregnancy was similar for IBD and non-IBD mothers, 13.7 (6.9) µg/m3 and 13.6 (4.2) µg/m3, respectively.IBD mothers with higher exposure of NO2 in the second and third trimester were at significant risk of preterm birth compared to non-IBD mothers [OR = 1.28 (CI 95%: 1.04-1.59) and OR = 1.23 (95% CI: 1.06-1.43), respectively]. The mean NO2 exposure was significantly higher in IBD mothers with preterm birth than in IBD mothers who delivered at term, at 19.58 (1.57) µg/m3 and 12.89 (6.37) µg/m3, respectively.Conclusions: NO2 exposure influenced the risk of preterm birth in IBD mothers. Higher risk of preterm birth in IBD was associated with higher exposure of NO2, suggesting vulnerability of preterm birth in IBD when exposed to NO2.