Impact of seasonality on gestational diabetes mellitus

Background: Gestational diabetes mellitus (GDM) is a major health concern that seems to be influenced by seasonality. Objective: To assess the impact of seasonality on the incidence of GDM in an Italian population. Method: This is a retrospective cohort study of 5,473 pregnant women attending the Operative Unit of Diabetes, who underwent GDM screening by means of the 75-g oral glucose tolerance test (OGTT), during the period from August 2011 to December 2016. Screening was performed at 16-18 or 24-28 weeks’ gestation, following the Health Italian Minister guidelines. All blood samples were undertaken in the Hospital itself, under the same temperature conditions, and analyzed in the nearby biochemical laboratory. Statistical analysis was performed by SPSS software. Results: 1,559 of 5,473 enrolled women (28.5%) were affected by GDM. The incidence of GDM was significantly higher during the summer season (33.7%) (P<0.001), and significantly lower during the winter (23.3%), compared with spring (P=0.035) and fall seasons (P=0.002). When the year was divided on a 24-hr temperature basis into two parts only, the warm half and the cold half, GDM was considerably lower in cold months compared to warm ones (P<0.0001). No difference was observed between the medians of fasting glycemia throughout the four seasons; instead, serum glucose levels at 1-h and 2-h after OGTT were higher in summer than in spring, autumn and winter. Results from multiple linear regression analysis supported the hypothesis that glucose levels at 1-h and 2-h following OGTT could be influenced by ambient temperature. Conclusion: Our data indicate that seasonal changes may influence variations in glucose tolerance during pregnancy, with GDM incidence increasing during the summer and declining during cold months.