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Despite a call to study the effect of weight gain pattern on development of gestational diabetes mellitus, few studies have correctly adjusted for independent effects of gain after the first trimester. We used a conditional percentile approach to model the independent association between first and second trimester weight gain trajectories and development of gestational diabetes.We sampled women delivering singleton infants from 1998 to 2010 at Magee-Womens Hospital in Pittsburgh, PA, (n = 124,590) using a case–cohort design. We modeled weight gain trajectories in the first and second trimesters of pregnancy using conditional weight gain percentiles, and used multivariable logistic regression to assess independent associations of the trajectory with gestational diabetes. We studied associations separately by prepregnancy body mass index category.The final cohort included 806 women with gestational diabetes and 4,819 randomly sampled women who delivered without gestational diabetes. In normal-weight women, every SD increase in weight gain in the first trimester above her predicted gain was associated with a 23% increased odds of gestational diabetes (95% confidence interval: 0.2%, 51%). Second trimester gain trajectory was not associated with gestational diabetes (odds ratio: 1.1, [95% confidence interval: 0.9, 1.3]) although the direction of effect was positive. This pattern was similar in obese class I and II but not in overweight and obese class III women.An upward weight gain trajectory in the first trimester was positively associated with gestational diabetes for women of most prepregnancy BMI categories. Second trimester weight gain trajectory was not associated with gestational diabetes for any group.