All About Glucose Screening and Tolerance Tests in Pregnancy

Among the tests that may be offered to you during pregnancy are the blood glucose screening and tolerance tests. These tests assess your body’s response to glucose, and they can give your doctor important information about whether you have gestational diabetes or if you may be at risk. Read on to find out more.

What Is the Difference Between Blood Glucose Screening and Tolerance Tests?

Glucose screening and tolerance tests help healthcare providers check the way your body processes glucose (sugar). The screening test is used to determine if you are at risk of having gestational diabetes, and the tolerance test can help determine whether you actually have the condition. This information, and the steps taken in response, are crucial for the health of you and your baby. You’ll need to have your blood drawn, so you might feel dizzy or lightheaded, and you may have slight bruising or bleeding around the spot where blood was drawn, but otherwise the risks associated with these tests are minimal.


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Why Might My Healthcare Provider Recommend These Tests?

Some doctors recommend all pregnant women — even those with a low risk of gestational diabetes — take a glucose screening test between weeks 24 and 28 of pregnancy. Some doctors, though, may not automatically offer it to pregnant women under age 25 who have a low risk of gestational diabetes. If you have one or more of the risk factors for gestational diabetes, your healthcare provider will likely offer you the screening or the glucose tolerance test at one of your first prenatal visits instead of waiting until the second trimester screening. Risk factors for gestational diabetes include:

  • Having had gestational diabetes in an earlier pregnancy

  • A family history of diabetes

  • Being obese

  • Having conditions such as metabolic syndrome or polycystic ovary syndrome (PCOS).

Pregnancy Glucose Test Procedures

The process is a little different depending on whether you’re having the screening or tolerance test; your doctor will be able to give you instructions for preparing for the glucose test you’re having. Read on to find out how each one happens:

Glucose Screening Test

The routine screening test, sometimes called the glucose challenge test, involves drinking a syrupy glucose drink at the clinic or lab. An hour later, you’ll have a blood test to measure your blood glucose levels. A blood glucose level below 140 mg/dL (7.8 mmol/L) is usually considered normal. If your doctor has concerns about the result, a glucose tolerance test may be recommended, because a higher blood sugar level means you are at a greater risk for gestational diabetes.

Glucose Tolerance Test

Depending on the instructions you receive from your healthcare provider, for 8 to 14 hours before the glucose test you should not eat or drink anything (other than small sips of water). When you get to the doctor’s office, the first blood sample you'll provide is a fasting glucose test to assess your blood sugar levels before you’ve had anything to eat or drink. Then, you will be given a syrupy drink, which may be easy or hard for you to consume, depending on how much of a sweet tooth you have! After this, you will have as many as three blood tests. The one hour glucose test takes place one hour after you’ve had the drink; the two hour test happens two hours after the drink; and one hour after that, you will have the three hour glucose test. It may be the most convenient to schedule a morning appointment so you can fast overnight. You may need to stay at the doctor’s office or lab for the full three hours, so take a book or have your phone fully charged. After the test, there’s nothing more for you to do, and you can get on with your day.

Pregnancy Glucose Tolerance Test Results

As the normal range of glucose tolerance test results may vary depending on the lab or clinic, your doctor is the best resource for information and advice on your specific results and recommended next steps. However, these general guidelines may apply:

  • If any one of the test results are higher than normal, a new test may be recommended for about four weeks later.

  • If two or more of the results are higher than normal, your doctor may diagnose you with gestational diabetes.

If You Are Diagnosed With Gestational Diabetes

Gestational diabetes is a form of diabetes that can affect some pregnant women. It happens when too much glucose stays in the blood instead of being used as energy. Gestational diabetes goes away after you give birth, and your doctor will advise you on steps to take to control the diabetes while you are pregnant. You will likely attend more frequent prenatal visits. Your provider may suggest lifestyle changes (like adopting healthy eating and moderate exercise) or medication to control your blood sugar levels, and you may need to periodically test your own blood sugar levels. You may also need to have regular glucose tests after your baby is born. It’s daunting to receive this diagnosis, but it may help you to read more about how to handle gestational diabetes so that you have a healthy pregnancy and baby.

FAQs at a Glance

Do I need to prepare before the screening or tolerance test? In general, it’s ok to eat and drink normally up until the day of your test. The tolerance test will likely require you to fast for several hours beforehand, but your healthcare provider will give you more detailed instructions. For the screening or challenge test, you won’t need to do anything to prepare. Will I be tested again later in pregnancy? You’ll need to keep an eye on your blood glucose levels if you test positive for gestational diabetes. Your healthcare provider will probably want to see you more often, and later in pregnancy, your baby’s growth and wellness will be checked, especially if you’ve been prescribed medication for your condition. What does gestational diabetes mean for my baby? Gestational diabetes can cause a fetus to grow larger than normal (nine pounds or four kilograms or more), which can make labour and delivery difficult. Complications can include cesarean delivery, heavy bleeding, or even vaginal tearing. Babies born with gestational diabetes are more likely to be overweight or obese in childhood and have a higher likelihood of developing diabetes themselves.

Don’t feel intimidated by these blood tests. They are important tools for ensuring you and your baby are healthy. 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. Learn more about pregnancy health, fitness, nutrition, and more in our downloadable Pregnancy Guide.