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Background: Current evidence suggests that excess weight and obesity are important risk factors for urinary incontinence in women. However, limited data exist regarding the relationships among body mass index, physical activity, and urinary incontinence in women in their 20s., Objective: This study aimed to (1) compare prevalence rates of urinary incontinence and high body mass index in 2 cohorts of young women, (2) explore associations between changes in body mass index and urinary incontinence using analysis of combined data from the 2 cohorts, and (3) explore the associations between physical activity and urinary incontinence, with adjustment for body mass index., Study Design: Data were collected from 2 cohorts of young women in the Australian Longitudinal Study of Women's Health (n=16,065), born 17 years apart: 1973-1978 (cohort 1) and 1989-1995 (cohort 2). The women in both cohorts completed the surveys at age 18 to 23 years (T1), with follow-up 4 years later (age, 22-27 years; T2). Self-reported urinary incontinence and body mass index were assessed in both surveys. As physical activity was measured using different questions in cohort 1 at T1, self-reported physical activity data were from T2 only. A total of 9 body mass index transition categories (based on body mass index status at baseline and follow-up) and 4 physical activity categories were created to assess multivariate-adjusted prevalence ratios for urinary incontinence at T2, using Poisson regression., Results: Rates of obesity increased in both cohorts over 4 years, from 6.6%% (95% confidence interval, 6.1-7.2) to 10.4% (95% confidence interval, 9.7-11.0) in cohort 1 and from 11.7% (95% confidence interval, 11.0-12.4) to 19.6% (95% confidence interval, 18.7-20.5) in cohort 2. Compared with women who maintained normal body mass index at T1 and T2, the prevalence ratio for urinary incontinence among those with body mass index >30 at age 22 to 27 years was higher, regardless of body mass index category at age 18 to 23 years (prevalence ratio at T1: 1.39 for normal body mass index [95% confidence interval, 1.1-1.76]; 1.44 for overweight [95% confidence interval, 1.27-1.63]; and 1.51 for obese [95% confidence interval, 1.36-1.67]). In cohort 1, there was no relationship between physical activity and urinary incontinence. However, in cohort 2 there was an inverse dose-response relationship between physical activity and urinary incontinence., Conclusion: The strong association between obesity and urinary incontinence in young women is a public health concern, given that obesity rates are likely to increase further with age and parity. The potential mitigating effects of physical activity on the obesity-incontinence relationship merit further investigation., (Copyright © 2021 Elsevier Inc. All rights reserved.)