The available data suggest that the frequency of diabetes in pregnancy is highly variable, generally reflecting the underlying pattern of NIDDM in the particular population. Different ethnic groups in the same environmental setting experience widely variable risk. Impaired glucose tolerance is usually more prevalent than diabetes in women of childbearing age. Maternal age, overweight, parity, and family history of diabetes all predispose to gestational diabetes mellitus (GDM). Incidence of GDM is low in the absence of risk factors, suggesting that selective screening programs may be cost-effective. The worldwide epidemic of glucose intolerance predicted by the latest World Health Organization studies will undoubtedly increase the burden of GDM, especially in the developing countries.