OBJECTIVE To examine the effect of prior meal ingestion on the glucose, insulin, and C-peptide response to a 50-g glucose challenge test in the third trimester of pregnancy. RESEARCH DESIGN AND METHODS Ten pregnant women with gestational diabetes mellitus and 12 nondiabetic pregnant control subjects matched for age and weight underwent a 50-g glucose challenge test on three occasions within a 2-wk period, in random order. On one occasion the test was administered in the fasting state (fasting glucose challenge test), on a second occasion the test was administered 1 h after ingestion of a standard mixed meal (1-h postprandial study), and on a third occasion the test was administered 2 h after ingestion of a standard mixed meal (2-h postprandial study). RESULTS In the control subjects, the plasma glucose level 1 h after ingestion of 50 g of glucose was higher in the fasting study (7.8 ±0.4 mM, 7 of 12 subjects with glucose ≥7.8 mM) than in the 1-h postprandial study (6.7 ± 0.3 mM, 3 of 12 subjects with glucose ≥7.8 mM) and the 2-h postprandial study (6.3 ± 0.4 mM, 3 of 12 with glucose ≥7.8 mM) (P < 0.01). In the postprandial studies of control subjects, insulin and C-peptide levels were higher at the time of ingestion of the 50 g of glucose, but the early (1 h) insulin secretory response was less than in the fasting study. In the diabetic patients, glucose levels 1 h after 50-g glucose ingestion were similar in the fasting study (10.5 ± 0.4 mM, no subjects with glucose value P < 0.03). In contrast to the control subjects, the insulin secretory response to 50 g of oral glucose was greater in the two postprandial studies than in the fasting study. CONCLUSIONS We have reached the following conclusions. 1) In nondiabetic gravidas, plasma glucose concentrations 1 h after ingestion of a 50-g oral glucose load are higher if administered in the fasting state compared with the postprandial state. 2) During normal pregnancy the Staub-Traugott Effect, i.e., improved glucose disposal after successive glucose load administrations, occurs and appears to be caused by mechanisms other than enhanced insulin secretion with successive glucose loads. 3) The effect of the prandial state on plasma glucose response to the 50-g glucose challenge test used to screen for gestational diabetes mellitus may be of sufficient magnitude to significantly alter the operating characteristics, i.e., sensitivity and specificity, of this test.