The term 'gestational diabetes mellitus' is unsatisfactory as it refers to a heterogeneous group of women, including those with minimal abnormality of carbohydrate metabolism and those with undiagnosed type II diabetes. However, perinatal morbidity is increased even in the group of women who have only impaired glucose tolerance; the mothers are at increased risk of subsequent development of diabetes, and there may also be long-term implications for the offspring. Current research is aiming to define the blood glucose levels at which risks increase so that clinical management can be appropriately directed. When available, the criteria required to justify population screening in pregnancy should be satisfied. The glucose challenge and fasting glucose tests are the leading contenders as appropriate screening tests to determine who should have the diagnostic glucose tolerance test. However, until this is reviewed, the widely used scheme of risk factors as a screening method should continue, as it detects at least 50% of women with gestational diabetes.