The incidence of gestational diabetes mellitus (GDM) is quickly rising and is currently complicating approximately 10% of all pregnancies. GDM carries significant perinatal risks such as a large for gestational age infant, shoulder dystocia, caesarean section, and neonatal hypoglycemia. Recently, the long-term health implications for mother and child have become evident. When dietary interventions fail to normalize glycaemia, treatment with anti-diabetic medication is indicated. By far, insulin is the treatment of choice for GDM. However, insulin treatment is laborious and a significant burden to both patients and health care resources. Promising data from small and inconclusive studies suggest that the oral anti-diabetic drugs may be a patient-friendly, safe and cost-effective alternative treatment option for GDM. These data justify and warrant the effort of evaluating metformin/glibenclamide as treatment option for GDM in a study of appropriate size and design. The SUGAR-DIP study is a randomized controlled trial of the clinical and cost-effectiveness and safety of oral anti diabetic treatment for the treatment of GDM as compared to insulin treatment. Sample size is 808. Outcomes are pregnancy complications, patient experience, cost-effectiveness and glycemic control. The trial will be conducted within the Dutch Obstetric Research Consortium that has completed many nationwide trials before some of them in multidisciplinary collaboration between obstetricians and diabetologists.