Gestational diabetes mellitus (GDM) is carbohydrate intolerance with onset or first recognition in pregnancy. More often than not the intolerance abates between pregnancies but may recur. As well, up to 70% of affected women will manifest type 2 diabetes mellitus within 10 years thereafter. GDM is diagnosed with a glucose challenge at approximately 28 weeks gestation though there is no universally accepted protocol for the procedure or for interpreting its results. Morbidity increases for both mother and foetus in GDM affected pregnancies. Maternal and early infant morbidity can be ameliorated by returning the maternal glucose economy to within healthy limits. Diet, exercise and, if needed, insulin, are used therapeutically to this end. Beneficial effects later in the affected infants life are less well established. Thresholds and targets vary from place to place.