IntroductionChlamydia trachomatis is the most commonly reported sexually transmitted disease and although infection during pregnancy is associated with neonatal complications, long-term respiratory consequences are unknown. We aimed to determine whether C. trachomatis infection during pregnancy is associated with asthma-related symptoms across childhoodMethodsThis study among 2475 children and their mothers was embedded in a population-based prospective cohort study. Maternal urine samples were tested for C. trachomatis infection during pregnancy. Questionnaires provided information on childhood physician-attended lower respiratory tract infections and wheezing, and current asthma at age 10 years. Lung function was measured by spirometry at age 10 years.ResultsThe prevalence of C. trachomatis infection during pregnancy was 3.2% (78 out of 2475). C. trachomatis infection during pregnancy was not associated with lower respiratory tract infections until age 6 years, but was associated with a higher odds of wheezing in children until age 10 years (OR 1.50 (95% CI 1.10–2.03)). C. trachomatis infection during pregnancy was associated with an increased odds of asthma (OR 2.29 (95% CI 1.02–5.13)), and with a lower forced expiratory volume in 1 s/forced vital capacity and forced expiratory flow at 75% of forced vital capacity (z-score difference −0.28 (95% CI −0.52– −0.04) and −0.24 (95% CI −0.46– −0.01), respectively) in children at age 10 years. The observed associations were only partly explained by mode of delivery, gestational age at birth or birthweight.ConclusionsC. trachomatis infection during pregnancy is associated with increased odds of wheezing, asthma and impaired lung function. The causality of the observed associations and potential underlying mechanisms need to be explored.