What You Need to Know About Glucose Screening Tests During Pregnancy

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A glucose screening test is a routine test that takes place during pregnancy to check for gestational diabetes, a type of diabetes that presents in pregnancy. The test checks for higher-than-normal levels of glucose (blood sugar), which could mean that you have or are at risk for developing gestational diabetes.

For most people, a glucose test is usually done between weeks 24 and 28 of pregnancy. If you have had gestational diabetes in a previous pregnancy, or if your healthcare provider is concerned about your risk of developing gestational diabetes, the test may be performed earlier. 

Gestational diabetes can cause problems for you and your baby during pregnancy and after birth, but the risks can be reduced if the condition is detected early and managed well.

pregnant woman getting blood test

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How Common Is Gestational Diabetes?

Gestational diabetes occurs in around 6% to 9% of pregnancies in the United States. In fact, between 2000 and 2010, the occurrence of gestational diabetes in the United States increased by 56%.

Purpose of Test

Gestational diabetes occurs when your body can’t make the extra insulin needed during pregnancy.

Insulin, a hormone made in your pancreas, helps your body use glucose for energy and helps control your blood glucose levels. During pregnancy, higher levels of pregnancy hormones can interfere with insulin.

Usually, the body can make more insulin during pregnancy to keep blood sugar normal, but sometimes the body cannot make enough and blood sugar levels go up.

If insulin levels cannot increase sufficiently, rising blood sugar levels will eventually result in gestational diabetes. Untreated, gestational diabetes can lead to complications for both you and the baby. These may include:

  • Macrosomia (oversized baby)
  • Increased chance of cesarean section delivery
  • High blood pressure during pregnancy or preeclampsia
  • Low blood sugar (hypoglycemia) in the baby immediately after delivery
  • Stillbirth
  • For the child, risk of obesity and developing type 2 diabetes later in life
  • Increased risk of developing type 2 diabetes for the mother

Risk Factors for Gestational Diabetes

Many people who develop gestational diabetes have no risk factors, but your doctor may wish to perform screening tests earlier in pregnancy if you:

  • Are overweight or obese
  • Had gestational diabetes in a previous pregnancy
  • Had a very large baby (9 pounds or more) in a previous pregnancy
  • Have high blood pressure
  • Have a history of heart disease
  • Have polycystic ovary syndrome (PCOS)
  • Have a personal history of impaired glucose tolerance, HBA1C greater than or equal to 5.7, or impaired fasting glucose
  • Are a member of an ethnic group that has a high prevalence of type 2 diabetes, such as Hispanic American, African American, Native American, South or East Asian, or Pacific Islander
  • Have a family history of diabetes, especially in a first-degree relative
  • Are currently using steroids (like prednisone)
  • Are having twins or triplets (multiple gestation)
  • Have a previous unexplained loss of pregnancy or birth of a malformed infant

Test Types

Two tests are available to screen for gestational diabetes. Most people will get the oral glucose tolerance test, only following up with the three-hour glucose tolerance test if their initial results are out of range.

Oral Glucose Tolerance Test

The oral glucose tolerance test (or glucose challenge test) is a routine test carried out in pregnancy. It is far from definitive, so don’t worry if you get a call that you need to come back for a follow-up test.

When the test is carried out: During weeks 24 through 28 of pregnancy, or earlier if you are identified as “high risk”

How it’s carried out: There is nothing you need to do to prepare for this test. During the test, you will drink a sugary beverage containing 50 grams of glucose. Your doctor will draw a blood sample one hour later to see how efficiently your body processes the glucose. Side effects may include slight nausea from the sugary beverage.

What your results mean: If your one-hour plasma glucose level is greater than or equal to 140 milligrams per deciliter of blood (mg/dL), gestational diabetes is suspected and further testing is recommended. If your one-hour plasma glucose level is 200 mg/dL or more, you may have type 2 diabetes.

Three-Hour Glucose Tolerance Test

This test is to confirm or rule out gestational diabetes.

When the test is carried out: After you have received an abnormal reading on the one-hour oral glucose tolerance test

How it’s carried out: You must fast for at least 8 hours before the test. Be sure to discuss any medications you are taking with your doctor to see if they may interfere with the test results. This test is similar to the one-hour oral glucose tolerance test, except the sugary beverage now contains 100 grams of glucose, not 50g. Blood is drawn before you drink the sugary liquid. This is called your fasting glucose level. Blood is drawn again after one hour, two hours, and three hours.

Test-taking tips:

  • Due to the long fast required, most people schedule the test first thing in the morning.
  • You may feel faint or lightheaded during this test. It is a good idea to have a designated driver, if possible, and to bring food to eat for when the test is over.

Abnormal Blood Values

Abnormal blood values for a three-hour glucose tolerance test:

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

Follow-Up

If one of your readings is abnormal, your doctor may suggest some dietary changes and possibly repeating the test later in your pregnancy. Two or more abnormal readings mean that you probably have gestational diabetes.

Managing gestational diabetes may involve:

  • Changes in your diet, specifically reduction and consistent intake of carbohydrates
  • Regular exercise
  • Insulin
  • Home glucose monitoring throughout the day
  • Additional tests to see how your baby is developing
  • Oral medications, such as metformin

Fortunately, blood sugar levels often return to normal within six weeks of delivery. However, having gestational diabetes can indicate an increased risk of developing type 2 diabetes in the future. Maintaining a healthy body weight through a careful diet and regular exercise can help reduce this risk.

A Word From Verywell

If you are pregnant, your obstetrician has probably told you that you will need gestational diabetes testing. Don’t worry—gestational diabetes testing is an important part of routine prenatal care. Because gestational diabetes can hurt you and your baby, it is critical to identify and treat the condition quickly.

If you are diagnosed with gestational diabetes, healthy eating, regular glucose monitoring, and maintaining an active lifestyle are effective ways to manage this condition. Sticking with your treatment plan should enable a healthy pregnancy and birth.

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Article 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.
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  2. Centers for Disease Control and Prevention. Diabetes during pregnancy. Updated June 12, 2018.

  3. American Diabetes Association. Gestational diabetes - symptoms, treatments.

  4. March of Dimes. Gestational diabetes. Updated April 2019.

  5. National Institute of Diabetes and Digestive and Kidney Diseases. Tests & diagnosis for gestational diabetes. Updated May 2017.

  6. MedlinePlus. Glucose screening tests during pregnancy. Updated March 2020.

  7. American Diabetes Association. How to treat gestational diabetes.