Reference values for the oral glucose tolerance test at each trimester of pregnancy.

A 100-gram oral glucose tolerance test (OGTT) was performed on a selected group of normal women at each trimester of pregnancy to establish reference values for hyperglycemia and hypoglycemia. Ninety three OGTT were performed in the first trimester, 121 in the second trimester, and 98 in the last trimester. The fasting serum glucose did not differ significantly between the trimesters. The values at 60 and 120 minutes were significantly different for the fifth, fiftieth, and ninety-fifth percentiles between each trimester of pregnancy. For the 180-minute readings, the fifth and fiftieth percentiles were not significantly different between the first and second trimester of pregnancy, but the results of the third trimester were significantly higher than those of the other trimesters. The mean fasting insulinemia remained relatively constant during pregnancy. Insulin response to OGTT increased during the progression of the pregnancy. The interpretation of the glucose tolerance tests during pregnancy, either to detect gestational diabetes or hypoglycemia, should take these physiologic changes into account.