BackgroundIt is unknown whether women with pregnancy-onset asthma are predisposed to worse pregnancy outcomes compared to women with pre-pregnancy asthma. ObjectiveTo explore whether pregnancy-onset asthma leads to worse perinatal outcomes compared with pre-pregnancy asthma. MethodsWomen who were discharged with a diagnosis of asthma and gave birth to a live singleton were included in this retrospective cohort analysis. Women were separated into groups based on whether the asthma was diagnosed during or before pregnancy. Clinical characteristics, perinatal outcomes, and asthma exacerbations (AE) between the two groups were compared. Results335 women were included in this study, of whom 39 (11.6%) had pregnancy-onset asthma and 296 had pre-pregnancy asthma. All the pregnant women in the pregnancy-onset group experienced asthma exacerbations (AE) during pregnancy. The proportion of chronic hypertension, chronic hypertension with superimposed preeclampsia, and spontaneous preterm births in the pregnancy-onset group was significantly higher than that in the pre-pregnancy asthma group. After adjusting for age, BMI, onset of asthma during pregnancy, and severity of AE through multivariate analysis, pregnancy-onset asthma was an independent risk factor for spontaneous preterm birth (aOR 7.71, 95% CI 1.30–46.12), severe AE was an independent risk factor for gestational hypertension and preeclampsia (aOR 3.58, 95% CI 1.30–9.87). ConclusionsDuring pregnancy, pregnancy-onset asthma in women is associated with an exacerbation of the condition. Obstetricians should be vigilant for signs of asthma onset during pregnancy. Other health-care providers should watch out for symptoms of gestational hypertension and preeclampsia in pregnant women with pre-existing or new-onset asthma.