Aim: Our aim in this study was to investigate the effect of maternal obesity and gestational diabetes mellitus (GDM) on cord blood dynamic thiol/disulfide homeostasis. Methods: A prospective case-control study was carried out in 125 pregnant women (27 GDM, 30 obese, 68 controls). Cord blood samples were collected from all participants and native thiol-disulfide exchanges were examined with automated method enabling the measurement of both sides of thiol-disulfide balance. Results: Disulfide amounts, disulfide/native thiol and disulfide/total thiol ratios were increased (p < 0.001), while native thiol/total thiol was decreased in the cord blood of babies born to an obese or diabetic mother (p < 0.001). Moreover, increased disulfide amounts, disulfide/native thiol, disulfide/totalthiol ratios and decreased native/total thiol were found to be significantly associated with adverse outcomes in GDM. Conclusion: The current study suggests that the offsprings born to obese or diabetic mothers are exposed to increased oxidative stress.