During the three previous days the subject must have an unrestricted diet (containing at least 150 g carbohydrate per day) and unlimited physical activity. The subject should remain seated during the test and should not smoke throughout the test. The test involves drinking a solution containing a certain amount of glucose, usually 75 g or 100 g, and drawing blood to measure glucose levels at the start and on set time intervals thereafter. The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria lead to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.The following are the values which the American Diabetes Association considers to be abnormal during the 100 g of glucose OGTT:

Fasting blood glucose level ≥95 mg/dL (5.33 mmol/L)
1 hour blood glucose level ≥180 mg/dL (10 mmol/L)
2 hour blood glucose level ≥155 mg/dL (8.6 mmol/L)
3 hour blood glucose level ≥140 mg/dL (7.8 mmol/L)An alternative test uses a 75 g glucose load and measures the blood glucose levels before and after one and two hours, using the same reference values.
Non-challenge blood glucose tests
When a plasma glucose level is found to be higher than 126 mg/dL (7.0 mmol/L) after fasting, or over 200 mg/dL (11.1 mmol/L) on any occasion, and if this is confirmed on a subsequent day, the diagnosis of GDM is made, and no further testing is required. These tests are typically performed at the first antenatal visit. They are simple to administer and inexpensive, but have a lower test performance compared to the other tests, with moderate sensitivity, low specificity and high false positive rates. Screening glucose challenge test
The screening glucose challenge test (sometimes called the OSullivan test) is performed between 24 and 28 weeks, and can be seen as a simplified version of the oral glucose tolerance test (OGTT). No previous fasting is required for this screening test, in contrast to the OGTT. The OSullivan test involves drinking a solution containing 50 grams of glucose, and measuring blood levels one hour later.If the cut-off point is set at 140 mg/dL (7.8 mmol/L), 80% of women with GDM will be detected. If this threshold for further testing is lowered to 130 mg/dL, 90% of GDM cases will be detected, but there will also be more women who will be subjected to a consequent OGTT unnecessarily. Oral glucose tolerance test
A standardized oral glucose tolerance test (OGTT) should be done in the morning after an overnight fast of between 8 and 14 hours.