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ImportanceAlthough certain air pollutants have been associated with adverse obstetric outcomes, evidence regarding the association of ozone (O3) exposure with the risk of hypertensive disorders in pregnancy (HDP) is limited and inconsistent.ObjectivesTo evaluate the association between gestational O3 exposure and HDP (ie, gestational hypertension and preeclampsia) risk, and to explore the window of susceptibility for O3 exposure during pregnancy.Design, Setting, and ParticipantsThis cohort study recruited pregnant patients from the Obstetrics and Gynecology Hospital of Fudan University in Shanghai, China, from March 2017 to December 2018. Participants were older than 18 years, had no infectious diseases or chronic noncommunicable diseases before pregnancy, were Shanghai residents with intent to participate in the study, and had plans to give birth in Shanghai. Gestational hypertension and preeclampsia were diagnosed according to the diagnostic criteria of the Chinese Society of Obstetrics and Gynecology during the study period. Data on residential addresses, demographic characteristics, and household living environments were collected from participants through a questionnaire survey. Data were analyzed from December 10, 2021, to May 10, 2022.ExposuresA high temporospatial resolution model was applied to predict individual levels of daily O3 exposure during pregnancy.Main Outcomes and MeasuresThe outcomes were gestational hypertension and preeclampsia, and data on these diagnoses were extracted from the hospital’s information system. A logistic regression model was used to estimate the associations between O3 exposure and risk of gestational hypertension or preeclampsia. Exposure-response associations were confirmed by restricted cubic spline functions. Distributed lag models were used to identify the O3 exposure window of susceptibility.ResultsAmong the 7841 participants (all females; mean [SD] age, 30.4 [3.8] years), 255 (3.2%) had gestational hypertension and 406 (5.2%) had preeclampsia. Pregnant individuals with HDP had considerably higher prepregnancy body mass indexes and lower educational levels. The mean (SD) O3 exposure levels were 97.66 (25.71) μg/m3 in the first trimester and 106.13 (22.13) μg/m3 in the second trimester. Each 10-μg/m3 increment of O3 exposure during the first trimester was associated with higher gestational hypertension risk (relative risk, 1.28; 95% CI, 1.04-1.57). However, gestational O3 exposure was not associated with the risk of preeclampsia. The restricted cubic spline function analysis revealed an exposure-response association between O3 exposure and risk of gestational hypertension.Conclusions and RelevanceResults of this study showed an association between increased gestational hypertension risk and O3 exposure during the first trimester. Furthermore, gestational weeks 1 to 9 were identified as the window of susceptibility for O3 exposure and elevated gestational hypertension risk. Sustainable O3 control is needed to reduce the disease burden of gestational hypertension.