pregnant woman having her blood drawn

Glucose Tests in Pregnancy: What to Know

By Erica Patino
Reviewed by Alyssa Quimby, M.D.
July 05, 2023

At around 24 to 28 weeks of pregnancy, your healthcare provider will have you come in for a recommended screening: the one-hour glucose challenge test. This checks for gestational diabetes, a blood sugar disorder that can develop during pregnancy.

Depending on the results, you may be asked to come back for a test called the three-hour glucose tolerance test. Here’s what you should know about glucose tests in pregnancy.

What Is the Glucose Challenge Test?

The glucose challenge test is a standard screening that is important for pregnant people. You usually don’t need to fast beforehand, and the appointment will take about one hour.

“Basically, we give you a drink with 50 grams of glucose [sugar] in it,” says Sunny Soroosh, a certified nurse-midwife at Lenox Hill Hospital in New York City. “We start a timer, and then an hour later, we draw your blood. Then we measure your blood sugar to see how well your body has processed it.”

If your results are below 135-140 mg/dL (milligrams per deciliter), this is expected, and you don’t need to do anything else.

If the results of your glucose challenge test are above 140 mg/dL, that doesn’t necessarily mean you have gestational diabetes. However, you will be asked to take a three-hour glucose tolerance test.

What Is the Glucose Tolerance Test?

The glucose tolerance test is a three-hour version of the test. It’s a diagnostic test, which means it can more accurately tell whether or not you have diabetes.

For some people, it shows that they have prediabetes. “There are women that fail the one-hour test but pass the three-hour test, which tells us their body doesn't handle sugar quite as well, although they are not diabetic,” says Teresa Hoffman, M.D.,” a board-certified ob-gyn at Hoffman and Associates OB/GYN in Baltimore.

Before the glucose tolerance test, you’ll be asked to not eat or drink anything for eight to 14 hours. You'll have blood drawn, and then be asked to drink a liquid with 100 grams of glucose. After the first hour, you'll have another blood draw. You'll have your blood drawn two more times, for a total of three hours and four blood draws.

Why Do I Need Glucose Tests During Pregnancy?

Up to 10% of pregnant people develop gestational diabetes. If it’s untreated, gestational diabetes can cause certain problems for your baby and your delivery. “This can include heart defects and macrosomia, which means a large baby,” says Soroosh. It can also cause difficulty with a vaginal delivery. It could cause low blood sugar in your baby after birth, which would need treatment and admission to the NICU (neonatal intensive care unit).

What Happens If I Have Gestational Diabetes?

If your glucose tolerance test shows you have diabetes—or prediabetes—your provider will refer you to a dietitian. This nutrition expert can teach you about following a diabetic diet, which means cutting down on carbohydrates and eating more protein, as well as being active.

If you have diabetes, you'll also check your blood sugar several times a day. “In addition, we do ultrasounds every four weeks to check on the growth of the baby,” says Hoffman. “If a person with gestational diabetes requires medication, we'll monitor them extra closely starting at around 32 to 34 weeks.”

Once you give birth, your blood sugar may return to normal. However, in about 50% of cases, people with gestational diabetes go on to develop type-2 diabetes, according to the CDC.

The good news is, gestational diabetes can be treated and risks can be reduced. It’s important to be tested for it at the time recommended by your provider, so you can get treatment if you need it.

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