Cooperation between the obstetrician and the physician for internal medicine is especially needed when a pregnant woman suffers from complex medical diseases. This contribution focuses on the special circumstances of maternal treatment also considering the fetal state with the following examples: drugs in the first trimester, cardiac diseases, pregnancy-induced hypertension, HELLP syndrome, gestational and type I diabetes, thrombosis, pulmonary diseases, infections, and treatment of cancer during pregnancy. Special obstetric requests to the physician are: keeping in close contact, avoiding recommendations for the mode of delivery, avoiding overtreatment of hypertensive disorders as well as intensifying the surveillance including the creation of preventive schedules for pregnant women with an increased risk of developing metabolic syndrome in their later life.