Why the Three-Hour Glucose Test Matters in Your Pregnancy

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Pregnancy is a time of significant physiological changes, some of which can potentially impact the health of both mother and baby. 

One of the conditions pregnant women may face is Gestational Diabetes Mellitus (GDM). 

The three-hour glucose test, also known as the oral glucose tolerance test (OGTT), is a critical tool in diagnosing GDM. Let’s understand the importance of this test and its role in managing a healthy pregnancy.

Understanding Gestational Diabetes

Gestational diabetes is a condition characterized by high blood sugar levels that first appear during pregnancy. It affects approximately 2-10% of pregnancies each year in the United States. 

Women with GDM have an increased risk of complications such as preeclampsia(High Blood Pressure in Pregnancy), and their babies are at higher risk of macrosomia (being large for gestational age), jaundice, and childhood obesity.

The Initial Screening: The One-Hour Glucose Challenge Test (CGT)

Most healthcare providers screen for gestational diabetes between 24 and 28 weeks of gestation. The initial screening test is often a one-hour glucose challenge test (GCT). 

In this test, the woman drinks water containing 50 grams of glucose;

  • One hour after you drink the solution, a blood sample is taken to measure your blood glucose level.
  • It’s called a “challenge” test because it’s challenging your body to process a sudden influx of glucose.
  • If your blood glucose level is below a certain threshold, usually 130 to 140 milligrams per deciliter (mg/dL), you’re considered to have passed the test. 

If your level is higher, it indicates that your body might not be processing glucose effectively and you may need a more definitive test, which is the three-hour glucose tolerance test.

Three-hour Glucose Tolerance Test (GTT):

  • This is a follow-up test if the one-hour GCT results are abnormal.
  • Before the test, you’ll be asked to fast overnight.
  • The test begins with a blood sample taken to measure your fasting blood glucose level.
  • You’ll then drink a solution containing a higher amount of glucose (100 grams).
  • Blood samples will be taken at one hour, two hours, and three hours after you drink the solution.
  • Each blood sample is used to see how your body is managing the glucose over time.
  • If at least two of the blood glucose readings are higher than the normal values established for each of the four time intervals, you are diagnosed with gestational diabetes.

The thresholds for a diagnosis of gestational diabetes on the three-hour glucose test in mg/dL are:

  • Fasting: 95 or higher
  • One hour: 180 or higher
  • Two hours: 155 or higher
  • Three hours: 140 or higher

If two or more of these readings are high, a diagnosis of gestational diabetes is made.

Why the Three-Hour Glucose Test Matters

The three-hour glucose test matters because it helps accurately diagnose or rule out gestational diabetes. Early diagnosis is crucial for managing gestational diabetes effectively and reducing the risk of complications.

Managing gestational diabetes typically involves monitoring blood sugar levels, adopting a healthy diet, exercising regularly, and in some cases, taking medication or insulin. 

Women with GDM are also monitored more closely during pregnancy and delivery to check for potential problems.

If gestational diabetes is not treated, it can lead to problems for the baby, such as being born at a very large size (macrosomia), which may lead to delivery complications; low blood glucose (hypoglycemia) at birth; and an increased risk of type 2 diabetes and obesity later in life.


Pregnancy is a time of joy, anticipation, and, naturally, some anxiety. Routine tests like the three-hour glucose test play a vital role in ensuring the health and well-being of both mother and baby. 

Despite the temporary discomfort, the test provides valuable information that enables healthcare providers to offer the best possible care throughout your pregnancy journey.

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