An imbalance between production of reactive oxygen species (ROS) and their clearance by antioxidant defence mechanisms results in the development of oxidative stress. Biological consequences of this state involve oxidative damage of key cellular components such as nucleic acids, lipids, or proteins and, in turn, impairment of cell and tissue function. Evidence from clinical and experimental studies supports the notion that oxidative stress is one of pathologic factors associated with gestational diabetes mellitus (GDM), metabolic disorder defined as any degree of glucose intolerance with onset or first recognition during pregnancy. It has been established that high blood glucose concentrations in diabetic pregnancy induce oxidative stress by several mechanisms, including an enhanced ROS production in mitochondria, the polyol pathway and the hexosamine pathway, as well as protein kinase C (PKC) activation and an advanced glycation end-products (AGEs) formation, and changes in biomarkers of free radical-induced damage and antioxidant defences have been detected in maternal diabetes. Moreover, hyperglycaemia-induced oxidative stress is related with some congenital anomalies in diabetic pregnancy. The current article provides an overview how oxidative stress is related to GDM, with special emphasis on the involvement of the hyperglycaemia-induced mechanisms in ROS overproduction, followed by discussion of indicators of oxidative stress. In addition, the relationship between oxidative stress and congenital malformations in diabetic pregnancy is described.