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Do you have a high risk for gestational diabetes? Doctors recommend that women predisposed to gestational diabetes undergo a 1-hour glucose tolerance test (GTT) during their second trimester. That’s because uncontrolled blood sugar levels during pregnancy can put the baby’s and the mother’s health at risk, causing complications such as excessive fetal growth. Through this GTT, your doctor can determine if your body has difficulty processing sugar, based on your blood sugar levels. Find out the normal range for a 1-hour glucose tolerance test in pregnancy, including tips to manage gestational diabetes until delivery.

Key Takeaways

  • The normal range for the 1-hour glucose tolerance test during pregnancy is below 140 mg/dL or 7.8 mmol/L.
  • A reading between 140 mg/dL and 190 mg/dL indicates elevated glucose levels, while more than 190 mg/dL means an abnormal GTT result that’s likely associated with gestational diabetes.
  • If you fail the 1-hour glucose tolerance test, your doctor will suggest the 3-hour gestational diabetes screening test.
  • Eating a low- or zero-carb diet, fasting, and exercising are some factors that may affect the glucose tolerance test and lead to inaccurate results.
  • Women with gestational diabetes are 8-10 times more likely to develop Type 2 diabetes.

What Is the 1-Hour Glucose Tolerance Test in Pregnancy?

The 1-hour glucose tolerance test (GTT) screens women for gestational diabetes — a type of diabetes that develops during pregnancy. The test is simple and involves the following steps:

    • A clinician takes a blood sample to measure the blood sugar level
    • Then, the patient drinks a glucose solution with 50 grams of sugar
    • After one hour, a second blood sample is taken to measure the blood sugar level again.

You may hear other people refer to this test using various terms, including:

    • Gestational diabetes screening
    • One-hour glucose challenge test
    • Glucose screening
    • Glucose loading test
    • Glucose load test

If the test result indicates high blood sugar levels, your healthcare provider will suggest the 3-hour version of the test. The process is the same as with the 1-hour glucose tolerance test, except you’ll need to fast for eight hours before the pre-test blood sample is taken.

When Is the 1-Hour GTT Done in Pregnancy?

Doctor checking pregnancy womans blood sugar levels

Gestational diabetes often develops around the 24th week of pregnancy. Your doctor may recommend a GTT test between 24 and 28 weeks of pregnancy, during the second to third trimester, or roughly between 5.5 and 6.5 months. Timing matters for accurate diagnosis.

If you’re at a higher risk for gestational diabetes, you may be tested early. Some factors that increase the risk of developing gestational diabetes include:

    • A prior gestational diabetes
    • A family history of Type 2 diabetes
    • Obesity or overweight
    • Being diagnosed with polycystic ovary syndrome (PCOS)

If you have any of these conditions, it’s critical to get tested to confirm a diagnosis early. Doing so improves treatment outcomes and helps sidestep serious risks.

Why Is a Glucose Tolerance Test Done During Pregnancy?

Taking a GTT during pregnancy is essential to avoid health complications associated with gestational diabetes. Uncontrolled maternal diabetes can cause macrosomia or excessive fetal growth, which can make labor and delivery more challenging for the mother. It can also cause breathing problems for the baby and increase the child’s risk of developing Type 2 diabetes and obesity.

One unique thing about gestational diabetes is that you can have it during your first pregnancy but not during the second or subsequent pregnancies, and vice versa.

In a study that looked at over 400,000 pregnant women in Canada, more than 10,000 had gestational diabetes only in their first pregnancy, and over 16,000 had it only in their second pregnancy.

Although the risk of gestational diabetes increases in women who have it during their first pregnancy, a GTT test in later pregnancies is still necessary to confirm a diagnosis.

What Is Considered a Failed 1-Hour Glucose Test?

If your result shows over 140 mg/dL, you fail the 1-hour glucose tolerance test. This reading indicates elevated or abnormal GTT in pregnancy that may signal gestational diabetes. In this case, the clinician may recommend rechecking your glucose using the 3-hour test.

Factors Affecting GTT Results

Some factors that may influence GTT results include:

    • Eating a low-carb diet: In the days leading up to the lab exam, you should eat a balanced diet with carbs. Many women make the mistake of reducing or eliminating carbs in their diet days before their bloodwork, which can result in inaccurate test results.
    • Fasting: The 1-hour glucose tolerance test in pregnancy doesn’t require fasting, so you can consume your meals as usual. Abstaining or skipping meals before the test may change your result.
    • Exercising before the test: Avoid intense physical activities, which may cause false positives.
    • Certain medications: Some medications, such as beta-blockers, statins, and corticosteroids, can raise blood sugar levels. Get personalized advice from your doctor about your medications.

What Happens If You Fail the 1-Hour Glucose Test?

Patient getting their blood sugar levels tested

If you fail the initial 1-hour glucose tolerance test — meaning your blood sugar level is over 140 mg/dL — your doctor will likely suggest a follow-up 3-hour glucose test to confirm gestational diabetes.

This 3-hour glucose test shows a longer and more detailed view of how your body processes sugar, helping doctors diagnose precisely. It’s also more specific, reducing the risk of overdiagnosis and false positives.

A day before your bloodwork, your doctor will ask you to fast for eight hours, meaning you shouldn’t eat or drink anything except water to ensure the accuracy of your results. Water doesn’t affect your blood sugar levels — drinking a few glasses while you fast is safe. Staying hydrated is essential to make the blood-drawing process easier.

Many prefer to do their 3-hour GTT in the morning as it’s easier to fast and control their appetite while sleeping. Here are things to expect for the second glucose test:

    • A doctor takes a blood sample to measure your fasting blood sugar levels.
    • Then, you drink a glucose solution that has 100 grams of sugar.
    • The doctor takes blood samples three times, after one, two, and three hours.

You’ll likely stay in the clinic until the test is finished.

Normal Range vs. Abnormal Range for the 3-Hour GTT

Here are the abnormal ranges of blood sugar level readings for the 3-hour GTT:

    • Fasting glucose levels: greater than or equal to 95 mg/dL (5.3 mmol/L)
    • 1-hour glucose levels: greater than or equal to 180 mg/dL (10.0 mmol/L)
    • 2-hour glucose levels: greater than or equal to 155 mg/dL (8.6 mmol/L)
    • 3-hour glucose levels: greater than or equal to 140 mg/dL (7.8 mmol/L)

You’ll likely pass the test if you don’t have two or more abnormal readings. Your doctor will interpret the results and confirm whether you have gestational diabetes.

Signs You May Have Passed Your Glucose Tolerance Test

Here are some signs you’ve passed your glucose test:

  • You Didn’t Hear Back From Your Doctor Immediately

    Sometimes, healthcare providers only call you if you have abnormal results. If you don’t hear back after a few days, you can contact the clinic to check your results.

  • You Don’t Experience Any Symptoms

    Often, gestational diabetes doesn’t have any symptoms, but some women may experience mild signs of elevated blood sugar. For instance, you may feel thirstier than usual or have to go to the toilet more often. Not experiencing any of these symptoms — and not hearing back from your doctor right away — likely means you’ve passed the test.

  • You Won’t Be Required to Undergo Any Further Testing

    If your GTT sugar levels in pregnancy are healthy, you’ll be informed that your results are normal. You won’t be required to go through another test in this case.

What If You’re Diagnosed With Gestational Diabetes?

If your 3-hour glucose tolerance test results indicate gestational diabetes, your doctor will develop a personalized management plan to ensure you and the baby remain safe until delivery.

An effective management plan includes healthy lifestyle changes, prescribed medication if needed, and regular prenatal care. Here are some general tips to minimize the risks of pregnancy complications.

  • Enlist a Dietitian’s Help

    Hire a registered dietitian nutritionist to help you customize a meal plan that provides optimal nutrition for you and the baby. They’re experts in medical nutrition therapy and can support you with carbohydrate tracking and personalized dietary changes.

  • Increase Physical Activity

    Exercise improves insulin sensitivity. Add low to moderate exercises such as walking, swimming, or yoga to your routine. However, before you start anything, ask your doctor which types of exercise are safe for you.

  • Take Your Medication as Prescribed

    If lifestyle changes aren’t enough to manage irregular blood sugar levels, your doctor may prescribe insulin or oral medications that are safe for you. Take these as prescribed to effectively manage your blood sugar and reduce risks.

  • Show Up for Prenatal Care Appointments

    Don’t miss your scheduled check-ups. Your physician can monitor your health and track changes throughout your pregnancy.

Pregnant Women Should Do a Glucose Tolerance Test 

Untreated or undetected gestational diabetes can lead to complications that compromise the safety of both mother and baby.

If you know you’re at risk of developing maternal diabetes, schedule a glucose tolerance test in your 24th week of pregnancy. The two-step test that includes the 1-hour and 3-hour GTTs can confirm gestational diabetes.

If your GTT results are positive, your doctor will customize a management plan to keep your blood sugar levels at healthy benchmarks until you give birth. This plan often encompasses lifestyle changes, medication, and prenatal care.

Make sure to attend your prenatal check-ups. If you experience any symptoms, inform your doctor so they can adjust the management plan.

Frequently Asked Questions

You can eat before the 1-hour glucose tolerance test, but avoid foods or beverages that can cause sugar spikes and affect your glucose readings. Eat a healthy, balanced meal that includes foods rich in fiber, protein, and healthy fats.

Yes, the 1-hour glucose tolerance test is generally safe for pregnant women. It’s recommended routinely for women at risk of gestational diabetes. The glucose solution you’ll drink during this test won’t affect your or your baby’s health.

A reading over 140 mg/dL is typically considered a failed result, indicating the need for additional testing. In this case, your doctor will suggest the 3-hour glucose tolerance test to determine whether you have gestational diabetes.

The healthy blood sugar level result for a 1-hour glucose tolerance test is 140 mg/dL or  7.8 mmol/L.

If your blood sugar level is over 140 mg/dL, you fail the 1-hour glucose tolerance test. The standard normal range can vary for each clinic. For instance, some healthcare providers may have a lower normal range, like 130-135 mg/dL. If you surpass this benchmark, they’ll recommend the 3-hour GTT to assess your risk for gestational diabetes.

Gestational diabetes is a strong precursor of Type 2 diabetes. A study revealed that women with gestational diabetes are 8-10 times more likely to develop Type 2 diabetes. They’re also twice as likely to be diagnosed with cardiovascular disease compared to women without gestational diabetes. If you have a history of maternal diabetes, get tested regularly for Type 2 diabetes to detect it early.
Kenneth Fill, PharmD, MBA

Kenneth Fill, PharmD, MBA, is a clinical pharmacist specialist and medical writer with a strong background in ambulatory care, pharmacogenomics, and medical content development. Based in the state of Michigan, he provides expert guidance on chronic disease state management. Kenneth completed his PharmD and MBA at the University of Toledo and pursued post-graduate residency training in ambulatory care and community-based pharmacy. His diverse background also includes academic detailing, clinical education, and interdisciplinary collaboration to enhance patient outcomes.