Guidelines for the 3-Hour Glucose Tolerance Test in Pregnancy
The 3-hour, 100-gram oral glucose tolerance test (OGTT) is used as the second step in the two-step approach to diagnose gestational diabetes mellitus (GDM), and requires at least two of four elevated values (fasting, 1-hour, 2-hour, and 3-hour) to confirm the diagnosis. 1
When to Perform the 3-Hour Test
The 3-hour OGTT is indicated when:
- A pregnant woman has a positive 50-gram, 1-hour glucose loading test (GLT) screening result at 24-28 weeks gestation 1
- The screening threshold is ≥140 mg/dL (7.8 mmol/L), though ACOG recommends ≥135 mg/dL (7.5 mmol/L) in high-risk ethnic populations 1
- Some experts use a lower threshold of ≥130 mg/dL (7.2 mmol/L) 1
Test Administration Protocol
The test must be performed after an overnight fast of at least 8 hours. 1
Blood glucose measurements are taken at four time points:
- Fasting (before glucose administration)
- 1 hour after 100-gram glucose load
- 2 hours after glucose load
- 3 hours after glucose load 1
Diagnostic Thresholds
GDM is diagnosed when at least two of the four values meet or exceed the following thresholds 1:
Carpenter/Coustan Criteria (Most Commonly Used):
- Fasting: ≥95 mg/dL (5.3 mmol/L)
- 1 hour: ≥180 mg/dL (10.0 mmol/L)
- 2 hours: ≥155 mg/dL (8.6 mmol/L)
- 3 hours: ≥140 mg/dL (7.8 mmol/L) 1
Alternative NDDG Criteria:
- Fasting: ≥105 mg/dL (5.8 mmol/L)
- 1 hour: ≥190 mg/dL (10.6 mmol/L)
- 2 hours: ≥165 mg/dL (9.2 mmol/L)
- 3 hours: ≥145 mg/dL (8.0 mmol/L) 1
Critical Clinical Considerations
A single abnormal value on the 3-hour test significantly increases risk for adverse outcomes. Women with only one elevated value have increased odds ratios for macrosomia (1.59), large-for-gestational-age infants (1.38), neonatal hypoglycemia (1.88), cesarean delivery (1.69), and pregnancy-induced hypertension (1.55) compared to women with normal results 2. These outcomes are comparable to women with diagnosed GDM 2.
ACOG now supports using a single elevated value for GDM diagnosis, recognizing the substantial risk associated with even one abnormal measurement 3, 4.
Two-Step vs. One-Step Approach Context
The two-step approach (which includes the 3-hour test) identifies approximately 5-6% of pregnant women with GDM, compared to 15-20% with the one-step 75-gram OGTT approach 1, 5. The two-step method has the advantage that the initial 50-gram screening does not require fasting, making it more convenient 1, 5.
Treatment of GDM diagnosed by the two-step approach reduces neonatal macrosomia, large-for-gestational-age births, and shoulder dystocia without increasing small-for-gestational-age births 1.
Post-Diagnosis Management
Once GDM is diagnosed:
- Immediate nutritional counseling and lifestyle modification should begin 4, 6
- Target glucose levels: fasting <95 mg/dL, 1-hour postprandial <140 mg/dL, 2-hour postprandial <120 mg/dL 4, 6
- Insulin therapy should be initiated if targets are not met within 1-2 weeks of lifestyle modification 6
- Approximately 70-85% of women achieve adequate control with lifestyle changes alone 4, 6
Postpartum Follow-Up
All women diagnosed with GDM must undergo a 75-gram OGTT at 4-12 weeks postpartum to assess for persistent diabetes 1, 4, 6. Women with a history of GDM have a 3.4-fold increased risk of developing type 2 diabetes and require lifelong screening at least every 3 years 4, 6.