Your doctor should assess your risk for gestational diabetes at your first prenatal visit. If you have a fasting blood sugar greater than 126 milligrams per deciliter (mg/dL) [7.0 mmol/L], or a random blood sugar greater than 200 mg/dL (11.1 mmol/L), that is confirmed on another day, you meet the criteria for the diagnosis of ]]>diabetes]]> and you do not need to be screened.

If you are at high risk of developing gestational diabetes—very ]]>obese]]> , prior history of gestational diabetes, glucose in your urine, or a strong family history of diabetes—your doctor may recommend that you undergo glucose testing as soon as possible. If your initial test is not positive you should be retested between 24 and 28 weeks of gestation.

Women who are of average risk should be given the 50-gram screening test between 24-28 weeks of gestation.

Newer guidelines do not require glucose testing for low-risk women, but to be low risk you must meet all of the following criteria: less than 25 years of age, normal weight before and during pregnancy, not a member of an ethnic group with a high risk of diabetes (Asian American, African American, Hispanic American, native American, or Pacific Islander), no known diabetes in any siblings or parents, no history of abnormal glucose tolerance, no glucose in urine, and no history of poor obstetric outcomes.

The 50-gram test is used to screen for gestational diabetes, while the 3-hour glucose tolerance test is used to diagnose gestational diabetes.

50-gram glucose screening test —You do not need to fast before this test. You are given a drink that contains 50 grams of glucose. One hour later, your blood is drawn to test your sugar level.

If your blood sugar level is less than 130 mg/dL (7.2 mmol/L) at the one-hour mark, the likelihood that you have gestational diabetes is low and you will not require further testing.

If your blood sugar level is greater than 130 mg/dL (7.2 mmol/L), the test is considered abnormal or positive. Your chance of having gestational diabetes mellitus is 90%. You will need to take the 3-hour oral glucose tolerance test to confirm the diagnosis. Not all women that have an abnormal or positive 50-gram glucose test have gestational diabetes.

100-gram diagnostic glucose tolerance test —Prior to this test, you will need to follow some special dietary restrictions.

  • Three days before the test—Eat a diet that contains at least 150 grams of carbohydrates a day.
  • For 10 to 14 hours before the test—Do not eat or drink anything, except water

The test is usually done in the morning in your doctor’s office. First, a blood sample is drawn to measure your fasting blood glucose level. Next, you will drink a glucose drink that contains 100 grams of glucose. The drink is very sweet; some people even feel a bit nauseated after drinking it. Blood samples are drawn every hour for three hours after you finish the drink.

The diagnostic values used by the American Diabetes Association are as follows:

TimeBlood glucose values
Fasting95 mg/dL (5.3 mmol/L)
1 hour180 mg/dL (10 mmol/L)
2 hours155 mg/dL (8.6 mmol/L)
3 hours140 mg/dL (7.8 mmol/L)

If two or more of your blood sugar levels are higher than the diagnostic blood glucose values, you have gestational diabetes. Once you have been diagnosed with gestational diabetes, your doctor will work with you to develop an appropriate treatment plan.

Other Tests

In addition to the above screening tests, additional tests may include:

  • Glycosylated hemoglobin (HbA1c)—a blood test which reflects the cumulative changes in your blood surgar over the last three months
  • Urine tests—protein (24 hour collection), ketones
  • Thyroid function tests: TSH and free thyroxine test
  • Tests to monitor the fetus— ]]>ultrasound]]> (usually done at 30-32 and 36-38 weeks), counting of fetal movement, amniotic fluid analyses