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Seasonal Changes in the Prevalence of Gestational Diabetes Mellitus. OBJECTIVE: To determine the effect of different seasons on the prevalence of gestational diabetes mellitus (GDM) by using World Health Organization criteria. RESEARCH DESIGN AND METHODS: The results of all pregnancy glucose tolerance tests (GTTs) were prospectively collected over a 3-year period in a temperate climate, and the results were grouped by season. RESULTS: The results of 7,369 pregnancy GTTs were available for consideration. In winter, the median 1-h and 2-h glucose results after GTT were significantly (P < 0.0001) lower than the overall 1-h and 2-h results. The prevalence of GDM at the 1-h diagnostic level was 29% higher in summer and 27% lower in winter than the overall prevalence (P = 0.02). The prevalence of GDM at the 2-h diagnostic level was 28% higher in summer and 31% lower in winter than the overall prevalence (P = 0.01). CONCLUSIONS: The prevalence of GDM varies according to seasons, which leads to the possible overdiagnosis of GDM in summer and/or underdiagnosis in winter. Further research into standardization of the GTT or seasonal adjustment of the results may need to be considered.