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Introduction: Low levels of cardiorespiratory fitness, moderate-to-vigorous-intensity physical activity (MVPA), and excess sedentary behaviors are associated with a greater risk of type 2 diabetes. Less is known about the role of fitness, MVPA, and sedentary behaviors before pregnancy with subsequent development of gestational diabetes mellitus (GDM), a strong risk factor for future diabetes and cardiovascular disease.Objective: To assess the associations of pre-pregnancy fitness, MVPA, and time spent watching television (a surrogate for sedentary behavior) with risk of GDM.Methods: Participants were 1,333 women enrolled in the Coronary Artery Risk Development in Young Adults (CARDIA) study who did not have diabetes at baseline (1985-86) or before post-baseline births. Baseline fitness was estimated using a graded symptom-limited maximal treadmill test and expressed in metabolic equivalent units (METS). Baseline MVPA (exercise units/day) was measured using the CARDIA physical activity history questionnaire, and television viewing (hours/day) was assessed by self-report in 1990-91. Logistic regression analysis was used to calculate odds ratios and 95% confidence intervals, adjusting for study center, baseline age, race, parity, education, family history of diabetes, smoking, alcohol, dietary fat, pre-pregnancy body mass index, HOMA-IR, HDL-cholesterol, and time from baseline to delivery.Results: Over 25 years of follow up, 164 women developed GDM. As shown in the Table, the odds of developing GDM were 22% lower for each 1 standard deviation increment in baseline fitness after adjustment (2.3 METS; OR 0.78, 95% CI: 0.65, 0.95, p=0.013). MVPA and television viewing were not statistically significantly associated with developing GDM.Conclusions: This is one of the first studies to report an inverse association between objectively measured pre-pregnancy fitness and subsequent development of GDM. Improved pre-conception fitness may benefit women at risk for GDM.