Seasonal variation and its interaction with pre-pregnancy BMI for GDM: a large population-based study in Tianjin, China

To evaluate the independent association of seasonal variation with GDM incidence in Tianjin, China, and to test whether there is an additive interaction between seasonal variation and pre-pregnancy body mass index (BMI) on GDM incidence. A population-based observational cohort study was conducted using the healthcare records data from Tianjin, China. Logistic regression was used to obtain odds ratios (ORs) and 95% confidence intervals (CIs). Additive interaction between pre-pregnancy BMI groups and seasons was estimated by using relative excess risk due to interaction (RERI), attributable proportion due to interaction (AP), and synergy index (S). Among the 112,639 pregnant women, 20.8% developed GDM at 24-28 weeks of gestation. The multivariable adjusted ORs and 95% CIs were 1.00, 1.00 (0.96-1.05), 1.15 (1.09-1.20) and 1.22 (1.16-1.29) respectively based on seasons (spring, summer, autumn and winter). Compared with the spring/summer and pre-pregnant BMI < 24 kg/m(2 )group, co-presence of autumn/winter and pre-pregnancy BMI >= 24 kg/m(2) increased the OR from 1.00 to 2.70 (95% CI 2.28-3.20), with a significant additive interaction: RERI (0.32, 95% CI 0.19-0.45), S (1.21, 95% CI 1.12-1.31) and AP (0.11, 95% CI 0.07-0.16). Autumn/winter is an independent risk factor for GDM incidence, and can significantly amplify the obesity-associated risk for GDM incidence. The underlying mechanism warrants further investigations. We suggest that seasonality is an additional factor when interpreting OGTT results for the diagnosis of GDM.