An oral glucose tolerance test is usually only done to diagnose gestational diabetes. For an OGTT, you must be in good health, not be taking any medications that might affect glucose, and be normally active. You should not take this test if you have been prescribed bed rest by your doctor. You should discuss this test with both your regular doctor and your obtetrician.
The test measures your blood glucose five times over a three-hour period. If your body's response to glucose is normal, then during the test your blood glucose will rise and then fall quickly. If you have diabetes, your blood glucose will rise higher than normal and stay high during the test.