Glucose Challenge and Tolerance Tests in Pregnancy

Among the tests that may be offered to you as part of your prenatal care during pregnancy are the glucose challenge and tolerance tests. These blood tests are used by your healthcare provider to screen for gestational diabetes, a type of diabetes that develops during pregnancy. Read on to find out what these blood tests entail, what if any prep you may need to do beforehand, and what the results mean.

What Is a Glucose Test in Pregnancy?

There are two glucose tests that are typically offered during pregnancy as part of your prenatal care. Both the glucose challenge test (the most common glucose screening test) and the glucose tolerance test measure how your body responds to glucose (sugar) and help determine if you have gestational diabetes, which may develop during your pregnancy. These glucose tests are routinely done during pregnancy and are recommended by the American College of Obstetricians and Gynecologists (ACOG) for all pregnant women.

When Do You Have the Glucose Tests During Your Pregnancy?

The glucose challenge test is usually performed between 24 and 28 weeks of pregnancy in women who are determined by their healthcare providers to be at low risk of gestational diabetes. If your healthcare provider has determined that you are at high risk of developing gestational diabetes, the provider may order the glucose challenge test for you earlier than the aforementioned range. If the result of the glucose challenge test comes back indicating a high level of glucose, your provider may order another test—called a glucose tolerance test—to more accurately diagnose your chance of developing gestational diabetes during pregnancy. If the result of an early glucose challenge test (for high-risk pregnant women) comes back indicating a normal level of glucose in your blood, your provider may order an additional glucose challenge test between 24 and 28 weeks of pregnancy.

What Makes You a High- or Low-Risk Candidate for Gestational Diabetes?

If you may be at high risk for gestational diabetes (sometimes initially suspected after a high glucose level is detected in your urine during a previous prenatal visit), your healthcare provider may also consider certain aspects of your medical history. The provider may take into account your pre-pregnancy weight (whether you were overweight or obese), your age, whether anyone in your family has had gestational diabetes or diabetes, your race or ethnic background (as certain populations are at higher risk), and outcomes of any previous pregnancies. Speak to your healthcare provider to find out whether you have a high risk or low risk of developing gestational diabetes. If the provider determines your risk is low, you may not have to take the glucose challenge test.

How Are the Glucose Challenge and Tolerance Tests Done?

Testing for gestational diabetes during your pregnancy may involve two tests. First, you'll have a glucose challenge test, and if the blood test results of that test show high levels of blood sugar, your healthcare provider may order a second test, the glucose tolerance test.

Glucose Challenge Test

The glucose challenge test is a one-hour procedure that begins with you drinking a sugary solution (typically 5 ounces of water with 50 grams of sugar). An hour later, a blood sample is taken and tested to measure your blood sugar level.

Glucose Tolerance Test

If the results of your glucose challenge test show levels that are higher than normal, your healthcare provider may order an additional test called a glucose tolerance test. This second, more complex test is a three-hour procedure that begins with a blood sample being drawn. Next, you drink a concentrated sugary solution (8 ounces of water with 100 grams of sugar). Then, your blood is drawn three more times. In total, four blood samples will be taken.

How Do You Prepare and What Can You Eat Before the Glucose Tests?

In preparation for either of the glucose tests, you can continue to eat normally in the days leading up to the test. There’s no specific prep you need to do or any reason for you to change your pregnancy. However, the glucose tolerance test does require you to fast beforehand. If you’re taking any medications, let your healthcare provider know in advance as some medications may affect test results.

Prep Before the Glucose Challenge Test

You don’t have to worry about what you can or cannot eat and drink before taking a glucose challenge test, because it’s a non-fasting, one-hour procedure that tests your blood sugar levels in pregnancy.

Prep Before the Glucose Tolerance Test

If your provider has scheduled a glucose tolerance test during your pregnancy, you will be given fasting instructions. This means you probably won’t be able to eat or drink anything except for small sips of water for about 8 to 14 hours before the test. You may find it convenient to schedule a morning appointment for the glucose tolerance test so that you can fast overnight. You will need to stay at the lab or clinic for the full procedure, so you might consider bringing something to read or do or having your phone fully charged so that you can occupy yourself during that time.

Are There Any Side Effects Associated With the Glucose Tests?

Most pregnant women don’t have any side effects during or after either glucose test. It's possible that you may feel nauseous, lightheaded, or even sweaty after drinking the glucose solution. It tastes like a very sweet soda, which can be easier to stomach for some women than for others. You’ll need to have your blood drawn, so you might feel dizzy, and you may have slight bruising around the blood withdrawal spot, but otherwise the risks associated with these tests are minimal.

What Do the Results of the Glucose Tests Mean?

As the normal range for blood sugar levels detected in the glucose challenge test (the one-hour procedure) and the glucose tolerance test (the three-hour procedure) may vary depending on the lab or clinic, your provider will be your best resource for information and advice on your specific results and recommended next steps in your pregnancy. However, these general guidelines may apply:

Glucose Challenge Test Results

What many medical experts consider a normal result for a glucose challenge test is a blood sugar level equal to or less than 140 mg/dL (7.8 mmol/L). The mg/dL means milligrams per deciliter and the mmol/L means millimoles per liter. This is how the glucose is measured in your blood sample. A normal result means you do not have gestational diabetes. If your level is higher than 140 mg/dL (7.8 mmol/L), your healthcare provider may recommend you do the glucose tolerance test, which can help better determine if you have gestational diabetes or not.

Glucose Tolerance Test Results

It’s worth knowing that approximately 66 percent of pregnant women who take the glucose tolerance test end up not being diagnosed with gestational diabetes. So, just because your provider has ordered a glucose tolerance test during your pregnancy doesn’t automatically mean you have gestational diabetes. So, what do the results of the glucose tolerance test indicate? Four blood samples are drawn during a glucose tolerance test. If just one of the four results is higher than normal, your provider may recommend that you make subtle changes to your current pregnancy diet. The provider may have you take another glucose tolerance test after you’ve made those changes to check if the results have improved. If more than one of the results is higher than normal, your provider may determine that you have gestational diabetes. The following figures indicate higher than normal blood sugar levels:

• Fasting blood sample: Higher than 95 mg/dL (5.3 mmol/L)

• 1-hour blood sample: Higher than 180 mg/dL (10.0 mmol/L)

• 2-hour blood sample: Higher than 155 mg/dL (8.6 mmol/L)

• 3-hour blood sample: Higher than 140 mg/dL (7.8 mmol/L).

If it’s determined that you have gestational diabetes, your healthcare provider can help guide you in making changes to help manage it. Eating a healthy diet, exercising during your pregnancy, and taking medication prescribed by your provider have been shown to help control blood sugar and to help keep you and your baby healthy.

The Bottom Line

The glucose challenge test is a routine procedure that’s recommended by medical experts during your pregnancy. It’s used to measure the glucose levels in your blood and make sure that they are normal. If by chance the results are determined to be high, your provider may recommend an additional test called the glucose tolerance test. The purpose of the glucose tolerance test is to better determine if you are at risk of developing gestational diabetes. This test is slightly more complex than the glucose challenge test, and you will have to fast beforehand. With the results of the glucose tolerance test in hand, your healthcare provider will be better able to diagnose you. If it’s determined you have gestational diabetes, know that it’s a manageable condition. Your provider can help you manage it by recommending a healthier diet, exercise, and possibly even medication. For some helpful information, take a look at our guides: Nutrition During Pregnancy and Exercising While Pregnant. Check with your healthcare provider before starting any exercise program during your pregnancy. Don’t feel intimidated by these tests or what the results mean. The tests are important tools for ensuring you and your baby are healthy throughout your pregnancy. If you’re in any doubt about these tests and what your results mean, your provider will be able to give you all the information you need.