The glucose challenge test is a blood test used to screen pregnant women for gestational diabetes. The test evaluates the ability to metabolize glucose (blood sugar), the body’s primary source of energy. A person with undiagnosed diabetes will have abnormally high glucose in the blood.

The American Diabetes Association recommends that all women be screened for diabetes during pregnancy. However, some physicians prefer to test only high-risk women, such as those who are overweight or have a family history of diabetes.

The glucose challenge test is typically ordered between the 24th and 28th weeks of pregnancy. Pregnant women who are at a particularly high risk of developing gestational diabetes may be screened earlier, between the 12th and 14th weeks of pregnancy.

During pregnancy, hormones from the placenta begin to temporarily block the action of the mother’s insulin. Gestational diabetes usually appears around the 24th week of pregnancy, a time when the placenta begins producing large quantities of these hormones. The glucose challenge test is used to determine if the mother’s body is able to overcome the insulin resistance on its own, or if glucose is building up in the blood, causing hyperglycemia. Excess glucose can lead to health problems for the mother and the fetus.

The terms “glucose challenge test” and “oral glucose tolerance test” (OGTT) are sometimes used interchangeably. However, the OGTT is a more involved blood test that requires fasting and other preparation. It can be used in diagnosing type 2 diabetes and impaired glucose tolerance (prediabetes) in addition to gestational diabetes.