Gestational diabetes screening and glucose tolerance testing (GTT)
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What it is
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Also, women who are older than 25 are at greater risk than younger individuals. Although a history of sugar in the urine is often included in the list of risk factors, this is not a reliable indicator of who will develop diabetes during pregnancy. Some pregnant women with perfectly normal blood sugar levels will occasionally have sugar detected in their urine.
The Council on Diabetes in Pregnancy of the American Diabetes Association strongly recommends that all pregnant women be screened for gestational diabetes. Several methods of screening exist. The most common is the 50 gram glucose screening test. No special preparation is necessary for this test, and there is no need to fast before the test.
50 gram glucose screening test, aka the 3-hour GTT
The test is performed by giving 50 grams of a glucose drink [usually in the form of a special can of orange or lemon-lime soda] and then measuring the blood sugar level one hour later. A woman with a blood sugar level of less than 140 milligrams per deciliter (mg/dl) at one hour is presumed not to have gestational diabetes and requires no further testing. If the blood sugar level is greater than 140 mg/dl the test is considered abnormal or "positive." Not all women with a positive screening test have diabetes. Consequently, a 3-hour glucose tolerance test must be performed to establish the diagnosis of gestational diabetes.
If your physician determines that you should take the complete 3-hour glucose tolerance test, you will be asked to follow some special instructions in preparation for the test. For three days before the test, eat a diet that contains at least 150 grams of carbohydrates each day. This can be accomplished by including one cup of pasta, two servings of fruit, four slices of bread, and three glasses of milk every day. For 10 to 14 hours before the test you should not eat and not drink anything but water. The test is usually done in the morning in your physician's office or in a laboratory.
First, a blood sample will be drawn to measure your fasting blood sugar level. Then you will be asked to drink a full bottle of a glucose drink (100 grams). This glucose drink is extremely sweet, and occasionally makes some people feel nauseated. Finally, blood samples will be drawn every hour for three hours after the glucose drink has been consumed. The normal values for this test are shown below.
| Diagnostic Criteria | Normal Mean Values | |
| Blood Glucose Level | Blood Glucose Level | |
| Fasting | 105 mg/dl | 80 mg/dl |
| 1 hour | 190 mg/dl | 120 mg/dl |
| 2 hour | 165 mg/dl | IOS mg/dl |
| 3 hour | 145 mg/dl | 90mg/dl |
From 752 Unselected Pregnancies
Early testing
If two or more of your blood sugar levels are higher than the diagnostic criteria, you have gestational diabetes. This testing is usually performed at the end of the second or the beginning of the third trimester (between the 24th and 28th weeks of pregnancy) when insulin resistance usually begins.
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If you had gestational diabetes in a previous pregnancy, or there is some reason why your physician is unusually concerned about your risk of developing gestational diabetes, you may be asked to take the 50 gram glucose screening test as early as the first trimester (before the 13th week).
Remember, merely having sugar in your urine or even having an abnormal blood sugar on the 50 gram glucose screening test does not necessarily mean you have gestational diabetes. The 3 hour glucose tolerance test must be abnormal before the diagnosis is made.
