The treatment of pregnant women with gestational diabetes mellitus (GDM) is an increasingly important field in obstetrical clinical practice. In recent years, the impact of GDM has been examined in terms of its psychological and medical treatment consequences. In this paper, we present a review of studies of the psychological outcomes of GDM and pre-existent diabetes. In addition, we describe the influence of GDM on health behaviour. Pre-existent diabetes is associated with decreases in well-being and should be recognized and treated by health care providers. Medical and psychological treatment should be offered in a specialized outpatient setting. In addition, diet and exercise counselling is recommended to give women the best control and responsibility for their health and that of their newborn.