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Context: Although recent trends in obesity have been well documented, generational patterns of obesity from early childhood through adulthood across birth cohorts, which account for the recent epidemic of childhood obesity, have not been well described. Such trends may have implications for the prevalence of obesity-associated conditions among population subgroups, including type 2 diabetes., Objective: Our objective was to evaluate trajectories of obesity over the life course for the US population, overall and by gender and race., Design, Setting and Participants: We conducted an age, period and birth cohort analysis of obesity for US individuals who participated in the National Health and Nutrition Examination Surveys (NHANES) (1971-2006)., Main Outcome Measures: Obesity was defined as a body mass index >or=95th percentile for individuals aged 2-16 years or >or=30 kg m(-2) among individuals older than 16 years. Age was represented by the age of the individual at each NHANES, period was defined by the year midpoint of each survey, and cohort was calculated by subtracting age from period., Results: Recent birth cohorts are becoming obese in greater proportions for a given age, and are experiencing a greater duration of obesity over their lifetime. For example, although the 1966-1975 and 1976-1985 birth cohorts had reached an estimated obesity prevalence of at least 20% by 20-29 years of age, this level was only reached by 30-39 years for the 1946-1955 and 1956-1965 birth cohorts, by 40-49 years for the 1936-1945 birth cohort and by 50-59 years of age for the 1926-1935 birth cohort. Trends are particularly pronounced for female compared with male, and black compared with white cohorts., Conclusions: The increasing cumulative exposure to excess weight over the lifetime of recent birth cohorts will likely have profound implications for future rates of type 2 diabetes, and mortality within the US population.