Prenatal Testing Basics: Glucose Screening and Glucose Tolerance Testing

What it is, who has it, when, why, and what it tells you.

By the editors of Parents magazine

When is the test taken?

Glucose screening is usually done between 24 and 28 weeks of pregnancy. If the test, which screens for gestational diabetes, reveals elevated levels of blood glucose, a glucose tolerance test is then given immediately thereafter.

Who needs to take the test, and why?

If you are over 30 (some doctors advise this test for women over 25), have a family history of diabetes, had a troubled earlier pregnancy, or are obese, you should be given a glucose screening during pregnancy. Even if you don't fit any of the above criteria, your practitioner may still advise this safe and simple test, because about half of women with gestational diabetes have no known risk factors. About 15 to 20 percent of women screened will show abnormal levels of glucose and will be given a glucose tolerance test. About 15 percent of the women given the second test will be diagnosed with gestational diabetes.

Gestational diabetes can be controlled by diet, exercise, or insulin, but if it goes undetected, the excess glucose (sugar) in the mother's blood raises the odds of the baby being macrosomic, or overly large. Macrosomic babies (weighing 9 pounds, 14 ounces or more) may have difficulty fitting through the birth canal and may have health problems such as jaundice, low blood calcium levels, or hypoglycemia. Luckily, by detecting gestational diabetes early, glucose screening and glucose tolerance testing give you a chance to minimize the risks of this condition.

What's involved?

For glucose screening, you will be given a glucose solution to drink. An hour after you've swallowed this syrupy, rather unpleasant beverage, you'll get a quick stick with a needle as a blood sample is taken from a vein in your arm. The glucose level in your blood is then analyzed. If it's found to be high, then you'll be asked to take the glucose tolerance test. This involves fasting overnight, then drinking a solution with an even higher dose of glucose. Your blood will then be sampled several times over a period of about three hours and tested for abnormal levels of glucose.

When are test results available, and how are they interpreted?

Test results are available immediately. If the test does reveal that you have gestational diabetes, you are not alone. It is one of the most common complications of pregnancy -- about three to five percent of all pregnant women in the United States have the condition. You may require to take special steps and to be closely monitored for the duration of your pregnancy. But take comfort in the fact that gestational diabetes almost always disappears immediately after birth.