The 3 hour OGTT is a diagnostic test for gestational diabetes. It measures glucose concentrations over three hours  after a 100-g oral glucose load. The test is done in the morning after an overnight fast of between 8 and 14 hours, and after at least 3 days of unrestricted diet (≥150 g carbohydrate per day) and physical activity. The test is performed on women not previously found to have overt diabetes or gestational diabetes mellitus (GDM) during early testing in their current pregnancy

At least two or more values must be equaled or exceeded for a positive diagnosis. A fasting plasma glucose > 7.0 mmol/L (126 mg/dl) is diagnostic of overt diabetes

These values represent the equivalent glucose oxidase plasma values calculated from the original data using the Somogyi-Nelson technique [3-5].

References:
1. O’Sullivan JB, Mahan CM. Criteria for the oral glucose tolerance test in pregnancy. Diabetes 13:278-85, 1964 PMID: 14166677
2. Carpenter MW, Coustan DR: Criteria for screening tests for gestational diabetes. Am J Obstet Gynecol 144:768-73, 1982.PMID: 7148898
3. Sacks DA, Abu-Fadil S, Greenspoon JS, Fotheringham N: Do the current standards for glucose tolerance testing represent a valid conversion of O’Sullivan’s original criteria? Am J Obstet Gynecol 161:638-41, 1989. PMID: 2782345
4.Vandorsten JP, NIH consensus development conference: diagnosing gestational diabetes mellitus.NIH Consens State Sci Statements. 2013 Mar 6;29(1):1-31.PMID:23748438
5. Gestational diabetes mellitus. Practice Bulletin No. 137. American College of Obstetricians and Gynecologists. Obstet Gynecol 2013; 122:406–16.PMID:23748438