When should the Carpenter‑Coustan criteria be used versus the National Diabetes Data Group (NDDG) criteria for diagnosing gestational diabetes?

When to Use Carpenter-Coustan vs NDDG Criteria

Use Carpenter-Coustan criteria exclusively for diagnosing gestational diabetes mellitus (GDM) in the two-step screening approach—the NDDG criteria are outdated and should not be used in clinical practice. 1, 2

Current Guideline Recommendations

The most recent ADA Standards of Care (2025) explicitly recommend only the Carpenter-Coustan criteria when using the two-step strategy for GDM diagnosis 2. The guidelines present two acceptable diagnostic strategies:

One-Step Strategy (IADPSG Criteria)

• Perform 75-g OGTT at 24-28 weeks
• Diagnose GDM if any one value meets or exceeds:
  • Fasting: 92 mg/dL
  • 1-hour: 180 mg/dL
  • 2-hour: 153 mg/dL

Two-Step Strategy (Carpenter-Coustan Only)

• Step 1: 50-g glucose load test (GLT) at 24-28 weeks
  • Proceed to Step 2 if ≥130,135, or 140 mg/dL at 1 hour
• Step 2: 100-g OGTT with Carpenter-Coustan thresholds
  • Diagnose GDM if at least two values meet or exceed:
    • Fasting: 95 mg/dL
    • 1-hour: 180 mg/dL
    • 2-hour: 155 mg/dL
    • 3-hour: 140 mg/dL

2, 3, 4

Why Carpenter-Coustan Supersedes NDDG

The Carpenter-Coustan criteria identify significantly more women at risk for adverse pregnancy outcomes compared to NDDG criteria. Research demonstrates that women diagnosed by Carpenter-Coustan but not by NDDG have:

• 1.70-fold increased risk of preeclampsia 5
• 1.44-fold increased risk of cesarean delivery 6
• 4.47-fold increased risk of macrosomia (>4500g) 6
• 2.24-fold increased risk of shoulder dystocia 6

The NDDG criteria use higher glucose thresholds (fasting 105 mg/dL vs 95 mg/dL; 2-hour 165 mg/dL vs 155 mg/dL), missing approximately 42-69% more women who would benefit from treatment 5, 6, 7. These "missed" women have the same long-term risk of developing type 2 diabetes as those diagnosed by NDDG criteria 7.

Clinical Decision Algorithm

For all pregnant women at 24-28 weeks gestation:

1. Choose your institutional approach (one-step vs two-step)

   • One-step: Use IADPSG criteria (preferred by ADA for optimizing gestational outcomes) 2
   • Two-step: Use Carpenter-Coustan criteria exclusively

2. Never use NDDG criteria in contemporary practice

   • Not mentioned in guidelines after 2015 1
   • Inferior sensitivity for detecting at-risk pregnancies 5, 6
   • No longer considered acceptable standard of care

3. For high-risk populations, consider:

   • Lower GLT threshold (130 mg/dL) for Step 1 screening 2
   • ACOG notes that even one elevated value on 100-g OGTT can be used for diagnosis 2

Important Caveats

The one-step IADPSG approach diagnoses approximately twice as many women with GDM compared to the two-step Carpenter-Coustan approach (15-20% vs 5-8% prevalence) 4, 8. However, 80-90% of women diagnosed by IADPSG criteria can be managed with lifestyle modifications alone 9, 10.

Do not use either Carpenter-Coustan or NDDG criteria for early pregnancy screening (<24 weeks)—these were derived from data at 24-28 weeks gestation and are not validated for first-trimester use 4.

For women requiring treatment, glycemic targets remain consistent regardless of diagnostic criteria used:

• Fasting <95 mg/dL
• 1-hour postprandial <140 mg/dL OR
• 2-hour postprandial <120 mg/dL

9, 10