Glucose Tests for Gestational Diabetes
Most women are screened for gestational diabetes between 24 and 28 weeks of pregnancy. Sometimes a test for diabetes is done earlier in your pregnancy if you are suspected of having pre-existing diabetes or have risk factors for gestational diabetes.
Blood samples drawn from a vein at timed intervals (see Test Preparation Needed)
Your test preparation depends on whether a one-step or two-step glucose tolerance test is done.
- One-step 2-hour oral glucose tolerance test: you will need to fast (usually overnight) before an initial blood sample is drawn. You will then be given a 75-gram dose of glucose to drink and your blood will be drawn at 1 hour and 2 hours after the dose.
- Glucose challenge test: you do not need to fast. You will be given a 50-gram glucose dose to drink and your blood is drawn after 1 hour.
- If the challenge test is abnormal, a 3-hour oral glucose tolerance test will be performed. You will need to fast before an initial blood sample is drawn. Then you will be given a 100-gram glucose dose and your blood drawn at 1 hour, 2 hours, and 3 hours after the dose.
Glucose is the primary energy source for the body's cells and the only energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood. Glucose tolerance tests help diagnose gestational diabetes, which is high blood glucose that develops during pregnancy.
Women diagnosed with gestational diabetes and their babies can receive appropriate treatment during pregnancy as well as after delivery. If untreated, mothers with gestational diabetes can pass more glucose to their developing babies than they need, and their fetuses may gain a lot of weight. These mothers may give birth to large babies (weighing 9 pounds or more), which can cause complications for the mothers and their babies, such as difficult labor and delivery, heavy bleeding after delivery, or need for a cesarean delivery. Women with gestational diabetes are more likely to have other conditions, such as high blood pressure. There is an increased risk of preeclampsia for women with gestational diabetes.
Babies born to mothers with gestational diabetes may experience more trauma during delivery due to their size. They may have other complications, such as breathing problems, jaundice, and may be born with low glucose levels. These babies also have an increased risk of being stillborn.
Both women who have had gestational diabetes and their babies have an increased risk of developing diabetes later in life.
Several health organizations recommend that all pregnant women should be screened for gestational diabetes using blood tests.
In the U.S., laboratory testing may involve either a one-step or two-step approach:
- One-step 2-hour oral glucose tolerance test (OGTT): after a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values (including the fasting glucose level) needs to be above a cutoff value for diagnosis of diabetes to be made.
- Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
- If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman's fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least 2 of the glucose levels at fasting, 1 hour, 2 hours, or 3 hours are above a certain level, then a diagnosis of gestational diabetes is made.
American College of Obstetricians and Gynecologists (ACOG) and the National Institutes of Health (NIH) consensus conference currently recommend the two-step approach, while the Endocrine Society recommends the one-step approach, and the American Diabetes Association (ADA) says either the one-step or two-step approach is appropriate.
How is the test used?
Glucose tolerance testing may be used to screen pregnant women for gestational diabetes. Several health organizations recommend a glucose tolerance test, using either a one-step or two-step approach. (See "What is being tested?")
In most cases, gestational diabetes will go away after delivery, but women who have gestational diabetes will be at an increased risk of having it again with subsequent pregnancies and of developing diabetes in the future. Some organizations, including ACOG and the ADA, recommend that women diagnosed with gestational diabetes be tested 4 to 12 weeks after delivery for diabetes that persists. A fasting blood glucose or hemoglobin A1c test may be used for this purpose.
When is it ordered?
Pregnant women are usually screened for gestational diabetes between their 24th and 28th week of pregnancy. You may be screened earlier in your pregnancy if you have signs and symptoms of diabetes or have had gestational diabetes with a previous pregnancy.
You may also be tested earlier in your pregnancy if you are at risk of type 2 diabetes (overt diabetes). You may be at risk if you:
- Are overweight, obese, or physically inactive
- Have a first degree relative (e.g., parent, sibling) with diabetes
- Delivered a baby weighing 9 pounds or more or had gestational diabetes with a previous pregnancy
- Have polycystic ovary syndrome
- Are of a high-risk race or ethnicity, such as African American, Hispanic, Native American, Asian American, Pacific Islander
- Have high blood pressure (hypertension) or taking medication for high blood pressure
- Have heart disease
What does the test result mean?
The following tables summarize the meaning of gestational diabetes glucose tolerance testing results.
Gestational Diabetes One-Step Approach
Samples drawn fasting and then 1 hour and 2 hours after a 75-gram glucose drink. Diagnosis of GDM is made when any of the values meet or exceed the limit. Time of sample collection glucose level Fasting Equal to or greater than 92 mg/dL (5.1 mmol/L) 1 hour Equal to or greater than 180 mg/dL (10.0 mmol/L) 2 hour Equal to or greater than 153 mg/dL (8.5 mmol/L)
Gestational Diabetes Two-Step Approach: Step One
Step One: Glucose Challenge Screen. Sample drawn 1 hour after a 50-gram glucose drink. Glucose Level Interpretation Less than 140* mg/dL (7.8 mmol/L) Normal screen 140* mg/dL (7.8 mmol/L) or greater Abnormal, needs OGTT (see Step Two below)
*Some experts recommend a cutoff of 130 mg/dL (7.2 mmol/L) because that identifies 90% of women with gestational diabetes, compared to 80% identified using the threshold of 140 mg/dL (7.8 mmol/L). ACOG recommends a lower threshold of 135 mg/dL (7.5 mmol/L) in high-risk ethnic groups with higher prevalence of gestational diabetes.
Gestational Diabetes Two-Step Approach: Step Two
Step Two: Diagnostic OGTT. Samples drawn at fasting and then 1, 2 and 3 hours after a 100-gram glucose drink. If two or more values meet or exceed the target level, gestational diabetes is diagnosed. Time of sample collection target levels** Fasting (prior to glucose load) 95 mg/dL (5.3 mmol/L) 1 hour after glucose load 180 mg/dL (10.0 mmol/L) 2 hours after glucose load 155 mg/dL (8.6 mmol/L) 3 hours after glucose load 140 mg/dL (7.8 mmol/L)
**Some labs may use different numbers.
How is gestational diabetes treated?
For many women with gestational diabetes, eating a healthy diet that is high in fiber and low in carbohydrates and getting regular exercise may be enough to lower blood glucose levels. However, some women may need diabetes medications. Diet control, regular exercise and/or diabetes medications such as insulin injections throughout the rest of the pregnancy may be required to bring and keep glucose levels to within normal levels. See the article on Diabetes or the links in Related Content for more on treatment.
If I am diagnosed with gestational diabetes, will I need to monitor my glucose levels?
If you have been diagnosed with gestational diabetes, your healthcare practitioner will recommend that you monitor your blood glucose levels to determine how far above or below normal your glucose is and to determine if you need diabetes medications.
Home glucose monitoring at regular intervals is typically recommended. This is usually done using a small instrument called a glucometer and testing strips. You would check your glucose by inserting the test strip into the glucometer and then placing a drop of blood from a skin prick onto the glucose strip. The glucometer then provides a digital readout of your blood glucose level. You will be given guidelines for how high or low your blood sugar should be at different times of the day. By checking your glucose regularly, you can see if the diet and medication schedule you are following is working properly for you.
Once I have gestational diabetes, will I continue to have diabetes after delivery?
Gestational diabetes usually goes away after you deliver your baby, but you will be at increased risk of developing diabetes later in your life. Some women have diabetes before becoming pregnant and don't know it. For these women, diabetes will not go away after delivering their baby.
Will I have gestational diabetes with subsequent pregnancies?