Using logistic regression and controlling for maternal first-trimester age, first-trimester BMI, and gravidity, we found that higher insulin secretion, estimated using the z-score of the Box-Cox transformed Stumvoll first-phase index, measured at ~26 weeks’ gestation, was significantly associated with a lower odds of GDM at the same assessment (odds ratio [OR] = 0.56 per SD increase of Stumvoll, 95% confidence interval [CI]: 0.39-0.80, p-value=0.001). This evidence concerns the gene INS and gestational diabetes.