Glucose Screening and Gestational Diabetes
During pregnancy, your care provider tests your blood glucose (sugar) level to evaluate your risk for gestational diabetes (a high blood sugar medical condition that can occur during pregnancy). Up to 10% of pregnant women develop gestational diabetes making it a common health issue during pregnancy. There are several health implications and risks as a result of gestational diabetes for you. This includes preeclampsia, postpartum low blood sugar, an increased risk of cesarean section, preterm labor and birth, and increased risk of gestational diabetes in subsequent pregnancies. For your baby, your risks include the possibility of having a macrosomic newborn (a newborn weighing 8 to 9 or more pounds), respiratory distress, and stillbirth. Care providers are unable to diagnose gestational diabetes without a blood sugar test. For this reason, your care provider will recommend testing your blood glucose levels during your second trimester.
How to Prepare for a Glucose Challenge Test (GCT)
You do not need to change your diet prior to the glucose screening test. The GCT is a screening test, which means it won't give you a diagnosis. Instead, it's designed to identify as many women as possible who might have gestational diabetes but need more testing to find out. So a positive result doesn't mean that you have gestational diabetes. In fact, only about a third of women who test positive on the glucose screening test actually have the condition. If you test positive, you'll need to take the glucose tolerance test (GTT) – a longer, more definitive test that tells you for sure whether you have gestational diabetes. Accordingly, please note that glucose testing can involve one-step or two-steps, depending on the results of your first blood draw.
What happens during the one-step glucose screening test?
You cannot consume any beverage or food at least 12 hours prior to the test. Your care provider may have more specific instructions about this timing - please ask them when you should refrain from drinking and eating the night before. When you arrive to your appointment, you will have your blood drawn. Then you will drink a liquid that contains glucose - it often tastes like a very sugary-sweet juice. After you drink this liquid, your blood will be drawn 1 hour later. A glucose screening level will be provided and your care provider will determine if the number is too high. If the number is too high, you will be invited back for a 3-hour glucose tolerance test.
Abnormal blood values for a 2-hour 75-gram oral glucose tolerance test are:
Fasting: greater than 92 mg/dL (5.1 mmol/L)
1 hour: greater than 180 mg/dL (10.0 mmol/L)
2 hour: greater than 153 mg/dL (8.5 mmol/L)
If only one of your blood glucose results in the oral glucose tolerance test is higher than normal, your provider may simply suggest you change some of the foods you eat. Then, your provider may test you again after you have changed your diet.
If more than one of your blood glucose results is higher than normal, you have gestational diabetes.
What happens during a 3-hour glucose tolerance test?
You cannot consume any beverage or food at least 12 hours prior to the test. Your care provider may have more specific instructions - please ask them when you should refrain from drinking and eating the night before. You will have your blood drawn first and then you will be asked to drink a glucose beverage. Your blood will be drawn every 60 minutes until you’ve completed a total of 2 additional blood draws. Your blood glucose level will be checked.
How to Prepare for the Test
For either the two-step test or one-step test, eat your normal food 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. Drinking the glucose solution is similar to drinking a very sweet soda. Some women may feel nauseated, sweaty, or lightheaded after they drink the glucose solution. Serious side effects from this test are very uncommon.
Why the Test is Performed
This test checks for gestational diabetes. Most pregnant women have a glucose screening test between 24 and 28 weeks of pregnancy. The test may be done earlier if you have a high glucose level 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
TWO-STEP TESTING
Most of the time, a normal result for the glucose screening test is a blood sugar that is equal to or less than 140 mg/dL (7.8 mmol/L) 1 hour after drinking the glucose solution. A normal result means you do not have gestational diabetes.
Note: mg/dL means milligrams per deciliter and mmol/L means millimoles per liter. These are two ways to indicate how much glucose is in the blood.
If your blood glucose is higher than 140 mg/dL (7.8 mmol/L), 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.
ONE-STEP TESTING
If your glucose level is lower than the abnormal results described below, you do not have gestational diabetes.
What Abnormal Results Mean
TWO-STEP TESTING
Abnormal blood values for a 3-hour 100-gram oral glucose tolerance test are:
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)
ONE-STEP TESTING
Abnormal blood values for a 2-hour 75-gram oral glucose tolerance test are:
Fasting: greater than 92 mg/dL (5.1 mmol/L)
1 hour: greater than 180 mg/dL (10.0 mmol/L)
2 hour: greater than 153 mg/dL (8.5 mmol/L)
If only one of your blood glucose results in the oral glucose tolerance test is higher than normal, your provider may simply suggest you change some of the foods you eat. Then, your provider may test you again after you have changed your diet.
If more than one of your blood glucose results is higher than normal, you have gestational diabetes.
Risks
You may have some of the symptoms listed above under the heading titled "How the Test will Feel."
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Taking a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
Excessive bleeding
Fainting or feeling lightheaded
Multiple punctures to locate veins
Hematoma (blood buildup under the skin)
Infection (a slight risk any time the skin is broken)