In pregnant women, diabetes mellitus (DM) can cause severe complications for both mother and child during pregnancy and delivery; for example, hypertension, pre-eclampsia, macrosomia or intrauterine fetal death. It is therefore essential to achieve good metabolic control in the mother from before conception to the postpartum period. A 35-year-old primipara with type 2 DM presented herself at our outpatient department at 21 weeks of gestation. Until this time her DM had been treated with oral antidiabetic drugs; these were withdrawn and conventional insulin therapy was initiated. Except for the first two weeks after insulin adjustment, blood glucose values were within the required range. Biometric tests performed until week 30 of gestation showed discreet fetal growth. In the week 31, fetal abdominal girth near the 95% limit was observed for the first time; this was soon followed by an explosion-like enlargement of the abdomen along with glycemic values at the lower limit. A cesarean section was performed in week 35 of gestation because of the excessive macrosomia. The female newborn had a birth weight of 4920 g and, one hour after delivery, a blood glucose of 10 mg/dl requiring an intravenous glucose bolus. In addition, the child needed oxygen and also needed both an enteral and a parenteral supply of glucose until day 7 after delivery. Mother and child were discharged from the clinic 19 days postpartum in good general condition. This case illustrates the complexity of treatment of glucose-tolerance disturbances during pregnancy and underlines the importance of fetal monitoring by ultrasound, given that measurement of maternal blood glucose does not always provide sufficient information on the metabolic situation of the fetus.