Screening for gestational diabetes mellitus (GDM) has been traditionally undertaken in the third trimester with the glucose challenge test (GCT) and this continues to be the current practice according to the American College of Obstetricians and Gynecologists. There were significant limitations to the GCT with a sensitivity of only 78% which would miss a significant number with GDM. Following the recommendations of the International Association of Diabetes and Pregnancy Study Group (IADPSG), the GCT has been abandoned and testing of all patients in now undertaken with the 75-gram 2-hour glucose tolerance test (GTT) at 24 – 28 weeks gestation. However, there are also problems with this program of testing for the detection of GDM. The 75-gram 1-hour GCT can also be administered in early pregnancy and has been found to be effective in screening not only for GDM but also for other abnormalities carbohydrate metabolism. We present our findings in 1500 patients who had both a 75-gram 1-hour GCT at the booking visit and a 75-gram 2-hour GTT in the third trimester. We have calculated the cut-off value for the a 75-gram 1-hour GCT in early pregnancy to be 6.0mmol/L (108mg/dL) with a sensitivity of 83.5% and a specificity of 49.2%. In a small proportion of patients with a GCT ≥10.0mmol/L (180mg/dL), an early GTT allowed the detection of pre-existing abnormalities of glucose metabolism associated with pregnancy. The false-positive GCT could also diagnose the disorder of gestational hyperglycemia and the false-negative GCT was able to detect mild GDM when the GTT was performed in the presence of risk-factors for GDM. We have concluded from our study that universal screening with the 75-gram 1-hour GCT in early pregnancy and also selective testing on the basis of risk-factors for GDM will detect the full range of abnormalities of carbohydrate metabolism encountered in pregnancy.