Gestational diabetes mellitus, however currently defined, is relatively rare in a UK Caucasian population, but is much more common in other ethnic groups. There is likely soon to be better agreement on diagnostic levels of hyperglycaemia in pregnancy, but there is still considerable reluctance to start insulin therapy. There is now good evidence that insulin administered twice daily during the third trimester to mothers who have even a mild degree of hyperglycaemia will reduce fetal size, and in particular fetal adiposity. In relation to recent concepts of the transgenerational passage of Type 2 diabetes and obesity, further epidemiological investigation is required. Insulin treatment in pregnancy may also prove to have a role in prevention of Type 2 diabetes in the next generation.