Diabetes mellitus - an impairment in the way in which the body manages glucose - is a condition which requires careful monitoring and control during pregnancy, because it can cause abnormalities in the foetus and problems for the mother. Usually none-diabetic women control their blood sugar very closely during pregnancy - the level lying between 4.5 and 5 millimoles per litre. Some women develop diabetes mellitus during the course of pregnancy, where it may be suspected if the level of blood glucose is greater than 7 millimoles per litre one hour after taking a drink containing 50 grams of glucose. The suspicion can be confirmed or denied by performing a test called a glucose tolerance test, where the patient is given 75 grams of glucose and the blood and urine are tested over a period of two hours.

Those whose results show frank diabetes mellitus need dietary advice and possibly insulin, while those with mildly abnormal results - called impaired glucose tolerance - may need to have the test repeated to ensure they do not go on to develop diabetes mellitus later in the pregnancy.