STUDY QUESTION: What are the respective roles of polycystic ovary syndrome (PCOS), long-term weight gain and obesity for the development of prediabetes or Type 2 diabetes mellitus (T2DM) by age 46 years? SUMMARY ANSWER: The risk of T2DM in women with PCOS is mainly due to overweight and obesity, although these two factors have a synergistic effect on the development of T2DM.WHAT IS KNOWN ALREADY: PCOS is associated with an increased risk of prediabetes and T2DM. However, the respective roles of PCOS per se and BMI for the development of T2DM have remained unclear.STUDY DESIGN, SIZE, DURATION: In a prospective, general population-based follow-up birth cohort 1966 (n = 5889), postal questionnaires were sent at ages 14 (95% answered), 31 (80% answered) and 46 years (72% answered). Questions about oligoamenorrhoea and hirsutism were asked at age 31 years, and a question about PCOS diagnosis at 46 years. Clinical examination and blood sampling were performed at 31 years in 3127 women, and at 46 years in 3280 women. A 2-h oral glucose tolerance test (OGTT) was performed at 46 years of age in 2780 women.PARTICIPANTS/MATERIALS, SETTING, METHODS: Women reporting both oligoamenorrhoea and hirsutism at age 31 years and/or diagnosis of PCOS by 46 years were considered as women with PCOS (n = 279). Women without any symptoms at 31 years and without PCOS diagnosis by 46 years were considered as controls (n = 1577). The level of glucose metabolism was classified according to the results of the OGTT and previous information of glucose metabolism status from the national drug and hospital discharge registers. LIMITATIONS, REASONS FOR CAUTION:The diagnosis of PCOS was based on self-reporting, and the questionnaire at 46 years did not distinguish between polycystic ovaries only in ultrasonography and the syndrome. Ovarian ultrasonography was not available to aid the diagnosis of PCOS. WIDER IMPLICATIONS OF THE FINDINGS:These results emphasize weight management already during adolescence and early adulthood to prevent the development of T2DM in women with PCOS, as the period between 14 and 31 years seems to be a crucial timewindow during which the women with PCOS who are destined to develop T2DM by 46 years of age experience a dramatic weight gain. Furthermore, our results support the view that, particularly in times of limited sources of healthcare systems, OGTT screening should be targeted to overweight/obese women with PCOS rather than to all women with PCOS.

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This work was supported by grants from the Finnish Medical Society Duodecim, the North Ostrobothnia Regional Fund, the Academy of Finland, the Sigrid Juselius Foundation, University Hospital Oulu and University of Oulu, the European Commission and the Medical Research Council, UK, Welcome Trust (089549/Z/09/Z). None of the authors have any conflict of interest.