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What is Involved in a Gestational Diabetes Test?

By Lindsey Rivas
Updated May 17, 2024
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The type of gestational diabetes test given to a pregnant woman varies by country, but the most common kind is an oral glucose screening test, followed by an oral glucose tolerance test. The screening test is typically given to a woman when she is 24 to 28 weeks pregnant. It may be given earlier if a woman is considered to be high risk, which includes being over age 35, obese, or having a family history of diabetes. If the first gestational diabetes test shows abnormal results, then the second test will be done to diagnose gestational diabetes. The second test, or glucose tolerance test, is necessary because a woman can have abnormal results from the screening test but not actually have gestational diabetes.

The first step in diagnosing gestational diabetes is for a woman to take an oral screening test. No fasting is required for this step, and the process takes about an hour. The pregnant woman will drink a liquid with about 50 grams of glucose, which generally tastes like sweet, noncarbonated soda. One possible side effect of the drink is nausea, and if vomiting occurs, the gestational diabetes test will have to be retaken on a different day. Approximately one hour after drinking the liquid, the pregnant woman will have blood drawn from her arm to test her blood sugar level, which will show how much of the glucose was metabolized by her body.

The level of blood sugar is measured in milligrams of glucose per deciliter of blood (mg/dL). What are considered to be abnormal results for the screening test can vary from doctor to doctor, but usually 140 to 200 mg/dL will indicate that the second gestational diabetes test should be given. Some doctors will consider abnormal to be as low as 130 mg/dL to catch gestational diabetes in more women. If the screening test results are over 200 mg/dL, many doctors will diagnose a woman with gestational diabetes without having to take the second test because of the extremely high blood sugar level.

If the second gestational diabetes test is deemed necessary, then a woman will commonly take the longer oral glucose tolerance test, which takes about three hours. For three days prior to the test, she should eat at least 150 grams of carbohydrates in addition to her normal diet. Fasting is required for eight hours before the test, although small sips of water are allowed.

The glucose tolerance test begins with drawing blood from a pregnant woman to record her blood sugar level after fasting. Next, she will drink a liquid with about 100 grams of glucose, which is more concentrated than the glucose drink for the previous screening test. Each hour for the next three hours, blood will be drawn from her arm to check her blood sugar level.

Abnormal blood sugar levels for the glucose tolerance test are commonly considered to be 180 mg/dL or more after waiting one hour, 155 mg/dL or more after two hours, and 140 mg/dL or more after three hours. If only one of the blood tests shows abnormal numbers, then a woman might have to be retested again later on in the pregnancy. If two or more blood tests have abnormal glucose levels, then the pregnant woman will be diagnosed with gestational diabetes.

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