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Glucose screening and tolerance tests during pregnancy

A glucose screening test is a routine test during pregnancy that checks a pregnant woman’s blood glucose (sugar) level.

A glucose tolerance test is done if a glucose screening test result is higher than normal. It is used to diagnose gestational diabetes .

Gestational diabetes is high blood sugar (diabetes ) that starts or is found during pregnancy.

Alternative Names

Oral glucose tolerance test - pregnancy (OGTT); Glucose challenge test - pregnancy

How the Test is Performed

For the glucose screening test:

  • You do not need to prepare or change your diet in any way.
  • You will be asked to drink a liquid that contains glucose.
  • Your blood will be drawn 1 hour after you drink the glucose solution to check your blood glucose level.

If your blood glucose is too high, you will need to come back for a glucose tolerance test. For this test:

  • Do not eat or drink anything (other than sips of water) for 8 - 14 hours before your test. (You also cannot eat during the test.)
  • You will be asked to drink a liquid that contains glucose.
  • You will have blood drawn before you drink the liquid, and again every 30 - 60 minutes after you drink it. Each time, your blood glucose level will be checked.
  • Allow at least 3 hours for this test.

How to Prepare for the Test

Eat your normal foods in the days before your test. Ask your health care provider if any of the medicines you take can affect your test results.

How the Test will Feel

Most women do not have side effects from the glucose tolerance test. Some may feel nauseated, sweaty, or light-headed after they drink the glucose solution. Serious side effects from this test are very uncommon.

Why the Test is Performed

Most pregnant women have a glucose screening test between 24 and 28 weeks of pregnancy. This test checks for gestational diabetes . The test may be done earlier if you have high glucose levels in your urine during your routine prenatal visits or if you have a high risk for diabetes.

Women who have a low risk for diabetes may not have the screening test. To be low-risk, all of these statements must be true:

  • You have never had a test that showed your blood glucose was higher than normal.
  • Your ethnic group has a low risk for diabetes.
  • You do not have any first-degree relatives (parent, sibling, or child) with diabetes.
  • You are younger than 25 years old and have a normal weight.
  • You have not had any bad outcomes during an earlier pregnancy.

Normal Results

Most of the time, a normal result for the glucose screening test is a blood sugar that is equal to or less than 140mg/dL 1 hour after drinking the glucose solution. A normal result means you do not have gestational diabetes.

Note: mg/dL means milligrams per deciliter. It is a way to indicate how much glucose is in the blood. 

If your blood glucose is higher than 140 mg/dL, the next step is the oral glucose tolerance test. This test will show if you have gestational diabetes. Most women (about 2 out of 3) who take this test do NOT have gestational diabetes.

What Abnormal Results Mean

Abnormal blood values for a 3-hour 100-gram oral glucose tolerance test are:

  • Fasting: greater than 95 mg/dL
  • 1 hour: greater than 180 mg/dL
  • 2 hour: greater than 155 mg/dL
  • 3 hour: greater than 140 mg/dL

If only 1 of your blood glucose results in the oral glucose tolerance test is higher than normal, your health care provider may simply suggest you change some of the foods you eat. Then, your health care provider may test you again after you have changed your diet.

If more than 1 of your blood glucose results is higher than normal, you have gestational diabetes.

References

Gestational diabetes mellitus. Practice Bulletin NO. 137. American College of Obstetricians and Gynecologists. Obstet Gynecol. 2013:122:406-16.

London MB, Catalano PM, Gabbe SG. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, PA: Saunders Elsevier; 2012:chap 39.

Updated: 6/11/2014

Cynthia D. White, MD, Fellow American College of Obstetricians and Gynecologists, Group Health Cooperative, Bellevue, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.


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