Glucose Tolerance Testing in Pregnancy
Perform gestational diabetes screening at 24-28 weeks of gestation using either a 75-gram oral glucose tolerance test (one-step approach) or a 50-gram glucose challenge test followed by 100-gram OGTT if positive (two-step approach). 1
Timing: When to Test
- Standard screening window: 24-28 weeks gestation for all pregnant women not previously diagnosed with diabetes 1, 2
- Early testing (first prenatal visit): Only for high-risk women with marked obesity, personal history of GDM, glycosuria, or strong family history of diabetes 3
- If negative at early screening, retest at 24-28 weeks 3
Glucose Load: Two Accepted Strategies
One-Step Approach (IADPSG/ADA Recommended)
Use a 75-gram glucose load with measurements at fasting, 1-hour, and 2-hour 1, 2, 4
- Patient must fast overnight (≥8 hours)
- GDM diagnosed if any one value meets or exceeds:
- Fasting: ≥92 mg/dL (5.1 mmol/L)
- 1-hour: ≥180 mg/dL (10.0 mmol/L)
- 2-hour: ≥153 mg/dL (8.5 mmol/L)
Two-Step Approach (NIH Consensus)
Step 1: 50-gram glucose challenge test (non-fasting) at 24-28 weeks 1, 5
- If 1-hour glucose ≥140 mg/dL (some use ≥130 or ≥135 mg/dL), proceed to Step 2
Step 2: 100-gram OGTT (fasting) 1, 5
- GDM diagnosed if at least two values meet or exceed (Carpenter/Coustan criteria):
- Fasting: ≥95 mg/dL (5.3 mmol/L)
- 1-hour: ≥180 mg/dL (10.0 mmol/L)
- 2-hour: ≥155 mg/dL (8.6 mmol/L)
- 3-hour: ≥140 mg/dL (7.8 mmol/L)
Key Clinical Considerations
Both approaches are endorsed by major guidelines, though they identify different populations 1, 2. The one-step 75-gram approach diagnoses more women with GDM (lower thresholds, only one abnormal value needed), while the two-step approach is more specific but requires two visits for those who screen positive.
Common pitfall: Testing too early (before 24 weeks) in average-risk women lacks evidence for benefit and may miss cases that develop later 6. Research shows glucose values and diagnostic thresholds differ between early and late pregnancy 7, 8.
For high-risk ethnic populations (Native American, Hispanic, Asian), ACOG recommends a lower screening threshold of 135 mg/dL or even 130 mg/dL for the 50-gram challenge test 1.
The choice between one-step and two-step approaches depends on your practice setting, patient population prevalence of GDM, and resource availability, but the 24-28 week timing is non-negotiable for standard screening 1, 6, 2.