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The study’s purpose was to examine the associations between exercise performed at different time points during pregnancy and gestational age (GA) in a population-based cohort study.Data included 61,098 singleton pregnancies enrolled between 2000 and 2006 in the Norwegian Mother and Child Cohort Study, conducted by the Norwegian Institute of Public Health. Self-reported exercise was collected from two questionnaires in pregnancy weeks 17 and 30. GA was determined on the basis of the expected date of delivery according to ultrasound, as registered in the Medical Birth Registry of Norway. We used logistic regression to analyze preterm (<37 completed weeks) and postterm births (≥42 wk). Comparison of mean GA by exercise levels was estimated by a general linear model.Mean GA for women exercising three to five times a week in week 17 was 39.51 (95% confidence interval [CI] = 39.48–39.54) compared with 39.34 (95% CI = 39.30–39.37) completed weeks for nonexercisers (P < 0.001). Mean differences remained for all categories of exercise after adjusting for confounding with the greatest mean difference between exercising three to five times per week in week 17 and nonexercisers (equals 1 d). Similar mean differences in GA were observed by exercise levels in week 30. The greatest protective effect on risk of preterm birth was observed for women exercising three to five times a week in week 17 or 30 (adjusted odds ratio (aOR) = 0.82, 95% CI = 0.73–0.91 and aOR = 0.74, 95% CI = 0.65–0.83, respectively) compared with nonexercisers. On the other hand, women exercising one to two or three to five times per week in week 17 were slightly more likely to have a postterm birth (aOR = 1.14, 95% CI = 1.04–1.24 and aOR = 1.15, 95% CI = 1.04–1.26, respectively). Mean GA did not differ by type of exercise performed during pregnancy.Exercise performed during pregnancy shifted the GA distribution slightly upward resulting in reduced preterm births and slightly increased postterm births.