What You Need to Know About Glucose Screening Tests During Pregnancy

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A glucose screening test is a standard test during pregnancy to help identify gestational diabetes. The test is done in two steps and checks a pregnant person's response to glucose (sugar). High glucose levels in the blood may indicate that you are at risk for or have gestational diabetes.

Most pregnant people will have a glucose screening test between weeks 24 and 28 of pregnancy. Your healthcare provider may ask you to have the test done earlier if you've been diagnosed with gestational diabetes in the past, are at risk for diabetes, or have high levels of glucose in your urine during routine prenatal checkups.

This article will discuss everything you need to know about the glucose screening test, including its purpose, risk factors, and the types of tests you may be asked to take.

pregnant woman getting blood test

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How Common Is Gestational Diabetes?

Approximately 10% of pregnancies in the United States are affected by gestational diabetes each year.

Purpose of Test

Gestational diabetes occurs when a pregnant person cannot make enough insulin or use insulin effectively. Insulin is a hormone produced by the pancreas. It's needed to move blood sugar (glucose) into cells throughout the body to be stored and later used as energy.

When a pregnant person has gestational diabetes, their pancreas continues to work and produce insulin, but it does not lower their blood sugar. The medical term for this condition is called insulin resistance.

Although the exact mechanism remains unknown, experts believe that the hormones produced by the placenta prevent the body from using insulin the way it should. In fact, a pregnant person may need up to three to four times as much insulin to effectively lower high glucose levels in the blood.

If insulin levels cannot rise to compensate for this, it will result in gestational diabetes. When left untreated, high glucose levels in the blood can cause problems for you and your developing baby. These may include:

  • Giving birth to a very large baby (over 9 pounds)
  • Increased chance of C-section
  • High blood pressure during pregnancy
  • Increased risk of postpartum depression
  • Premature birth
  • Birth trauma
  • Stillbirth
  • Low blood sugar in the baby after delivery
  • Increased risk of type 2 diabetes later in the baby's life

Because gestational diabetes doesn't typically cause noticeable symptoms, routine glucose screening tests are vital.

Risk Factors for Gestational Diabetes

All pregnant people are at risk for gestational diabetes. However, some factors may increase a person's risk. These include:

  • Being overweight or obese
  • Lack of physical activity
  • Having a family history of diabetes
  • People who are African-American, American Indian, Pacific Islander, Asian American, or Hispanic
  • Having prediabetes
  • Being over the age of 25
  • Having PCOS
  • Previously giving birth to a baby over 9 pounds
  • Having high blood pressure or heart disease

Test Types

There are two test types that may be given to show how well a pregnant person uses glucose. The oral glucose tolerance test is often given first, followed by the three-hour glucose tolerance test if results from the first test are higher than normal.

Oral Glucose Tolerance Test

The oral glucose tolerance test, also called the glucose challenge test, is the first glucose screening test carried out during pregnancy.

When the test is carried out: During weeks 24 to 28 of pregnancy, however, it may be done earlier if you are considered high risk.

How it's carried out: This test does not require you to fast. You'll be asked to drink a very sweet beverage that contains 50 grams of glucose within five minutes. After one hour, a blood sample is taken to measure the amount of glucose in your blood.

What the test results mean: If blood sugar levels are above 140 milligrams per deciliter (mg/dL), gestational diabetes is suspected, and you may have to return for a three-hour glucose tolerance test. If your blood sugar is over 200 milligrams per deciliter (mg/dL), you may have type 2 diabetes.

Three-Hour Glucose Tolerance Test

This test is carried out to confirm or rule out a diagnosis of gestational diabetes.

When the test is carried out: If your blood glucose levels from the first test are too high, your healthcare provider will ask you to return for the three-hour glucose tolerance test.

How it's carried out: This test requires you to fast for at least eight hours. You'll be asked to drink a sugary beverage that contains 100 grams of glucose. Blood samples will be taken before you drink the beverage and every hour after for three hours.

What the test results mean: If both of your glucose screening tests are abnormal, it means you probably have gestational diabetes.

Abnormal results from the three-hour glucose tolerance test are as follows:

  • Fasting: greater than 95 milligrams per deciliter
  • One hour: greater than 180 milligrams per deciliter
  • Two hour: greater than 155 milligrams per deciliter
  • Three hour: greater than 140 milligrams per deciliter


If one of your test results is abnormal but the other is normal, your healthcare provider may suggest diet and lifestyle changes. Once changes are implemented, they may ask you to repeat the test. If you receive abnormal results from more than one of your tests, you likely have gestational diabetes.

Your healthcare provider may begin treatment by asking you to make diet changes, exercise, and carefully monitor your blood glucose levels. If lifestyle modifications alone are not enough to control your blood sugar, you may need to take insulin, which is considered safe in pregnancy.

Managing gestational diabetes involves:

  • Checking blood sugar three to four times each day and keeping a log
  • Following a healthy diet that consists of three meals and two snacks each day while reducing your carbohydrate intake
  • Maintaining a healthy weight gain during pregnancy
  • Taking insulin or other diabetes medications
  • Performing 30 minutes of moderate-intensity activity at least five days a week

Gestational diabetes often goes away after pregnancy. However, it is likely to return in future pregnancies and can even develop into type 2 diabetes later in life. Maintaining a healthy body weight, exercising, and eating a healthy diet can help reduce this risk.


During pregnancy, a standard test to help identify gestational diabetes is a glucose screening test. The oral glucose tolerance test is given first, typically between 24 and 28 weeks of pregnancy, which requires drinking a very sweet beverage before your blood sugar levels are tested. If tests results are higher than normal, you will be asked to take a three-hour glucose tolerance test. Gestational diabetes can be managed, but it carries the risk of preterm birth, high blood pressure during pregnancy, increased chance of a C-section, and more if left untreated.

A Word From Verywell

If you are pregnant, your healthcare provider or obstetrician has likely told you that you will need testing for gestational diabetes. While the thought of it can be scary, it is an important part of routine prenatal care. The good news is gestational diabetes is manageable, and early detection can help keep you and your baby safe during pregnancy and after birth. If you are diagnosed with gestational diabetes, be sure to maintain an active lifestyle, follow a healthy diet, and closely monitor your glucose levels to ensure they stay in an optimal range.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. Glucose screening tests during pregnancy.

  2. American Diabetes Association. Gestational diabetes.

  3. Centers for Disease Control and Prevention. Gestational diabetes.

  4. American Diabetes Association. How gestational diabetes can impact your baby.

  5. March of Dimes. Gestational diabetes.

  6. Johns Hopkins Medicine. Gestational diabetes mellitus.

  7. National Institute of Diabetes and Digestive and Kidney Diseases. Tests & diagnosis for gestational diabetes.

  8. American Diabetes Association. Prenatal care.

By Lindsey DeSoto, RD, LD
Lindsey Desoto is a registered dietitian with experience working with clients to improve their diet for health-related reasons. She enjoys staying up to date on the latest research and translating nutrition science into practical eating advice to help others live healthier lives.