What's the difference between the one-hour glucose test and the three-hour test conducted during pregnancy? What are the "normal" and "abnormal" readings for these tests?

--Seema

These tests are used to look for gestational diabetes, which if untreated may lead to complications in both mother and baby. The one-hour glucose test is usually done between 24 and 28 weeks of pregnancy as a screening test. Any abnormal values need to be followed up with the more extensive three-hour test to see if there really is a problem. In the one-hour test, the woman drinks a flavored liquid that contains exactly 50 grams of glucose, and her blood is drawn one hour later. No special preparation is needed for this test.

The three-hour glucose tolerance test (GTT) is more extensive. First, the woman fasts 8-12 hours (typically overnight). Then blood is drawn four times: First, a baseline "fasting" sample is taken; then the woman ingests a drink containing 100 grams of glucose, and additional samples are taken one, two and three hours after that. The most commonly used cutoff for normal on the one-hour test is a glucose value of 140 mg/dl, although now more and more obstetricians are using 135 mg/dl or even 130 mg/dl as the "normal" value. If a result is higher than this, the three-hour GTT is performed. Doctors may use any of several references for the three-hour test. The two most common are the NDDG and the Carpenter scales. Normals for both are as follows:

No matter what criteria are used, the diagnosis of gestational diabetes requires that two of the readings be abnormal.

Therapy for gestational diabetes may merely be a matter of modifying the diet and ensuring regular exercise. Some women will require insulin injections. In either case, blood sugar levels are monitored to determine if the therapy is working. The goal is to keep the fasting blood glucose at less than 105 mg/dl and values two hours after eating at less than 120 mg/dl. About half of women with gestational diabetes will go on to develop overt diabetes later on in life -- that is, diabetes independent of pregnancy.

The American College of Obstetricians and Gynecologists publishes a pamphlet on gestational diabetes; your doctor may have this or other educational material for you to read.