Gestational diabetes mellitus (GDM) is one of the most common pregnancy complications. In Germany, the prevalence in 2019 was 6.8 %. The predisposition includes obesity, advanced age, familial diabetes burden, unfavourable lifestyle, e. g. lack of exercise and an unhealthy diet, as well as previous GDM. These factors are largely identical to the predisposition to type 2 diabetes mellitus. There is no uniform GDM phenotype; in addition to pregnant women with insulin resistance, there are those with beta-cell dysfunction and mixed types. Once pregnancy has been diagnosed, later GDM can be reduced by 27–55 % through ergometer training, a Mediterranean diet with supplements of olive oil and pistachios, and a more complex accompanying program. After pregnancy with GDM, women have a 6.5–10-fold increased risk of converting to type 2 diabetes within 5–25 years and a 2-fold increased risk of cardiovascular complications after 10 years. Only 40 % of women take part in follow-up care. Intensive lifestyle intervention with diet, exercise and weight control could effectively reduce the risk of diabetes in women with prediabetes. In addition, the use of metformin could be considered in high-risk cases. Hyperglycaemia in pregnancy is programming the children’s risk of overweight and obesity via epigenetic effects, which could be further increased by unfavourable familial lifestyle factors. Prevention programs would only be effective if health policy measures are implemented in addition to individual preventive programs.