What are the diagnostic criteria for gestational diabetes mellitus (GDM)?

Diagnostic Criteria for Gestational Diabetes Mellitus

Two diagnostic strategies exist for GDM, and you can use either the one-step approach (75g OGTT with any single abnormal value) or the two-step approach (50g screening followed by 100g OGTT with two abnormal values), both performed at 24-28 weeks of gestation. 1

Timing of Screening

• Screen all pregnant women at 24-28 weeks of gestation unless they have already been diagnosed with overt diabetes 1, 2
• Test immediately at the first prenatal visit if high-risk features are present: marked obesity (BMI ≥30 kg/m²), previous GDM history, glucosuria, strong family history of diabetes, or high-risk ethnicity (Asian, Hispanic, African American, Native American, Pacific Islander) 1, 2, 3
• If early screening is negative in high-risk women, repeat testing at 24-28 weeks 1, 4

One-Step Approach (IADPSG Criteria)

Perform a 75g oral glucose tolerance test after an overnight fast of at least 8 hours, measuring plasma glucose at fasting, 1 hour, and 2 hours 1, 2

Diagnosis requires ANY SINGLE value meeting or exceeding these thresholds: 1

• Fasting: ≥92 mg/dL (5.1 mmol/L)
• 1 hour: ≥180 mg/dL (10.0 mmol/L)
• 2 hours: ≥153 mg/dL (8.5 mmol/L)

This approach identifies approximately 15-20% of pregnant women with GDM, roughly 2-3 times more than the two-step approach 5, 6

Two-Step Approach (ACOG-Supported)

Step 1: Initial Screening

Perform a 50g glucose load test (nonfasting) with plasma glucose measured at 1 hour 1

• Proceed to diagnostic testing if glucose is ≥130 mg/dL, ≥135 mg/dL, or ≥140 mg/dL 1
• The 140 mg/dL threshold identifies approximately 80% of GDM cases; lowering to 130 mg/dL increases sensitivity to 90% 1

Step 2: Diagnostic Testing

Perform a 100g OGTT after an overnight fast, measuring plasma glucose at fasting, 1 hour, 2 hours, and 3 hours 1

Diagnosis requires AT LEAST TWO values meeting or exceeding these thresholds (Carpenter-Coustan criteria): 1, 2

• 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)

Important caveat: ACOG recently noted that a single elevated value (rather than two) may be used for diagnosis, which significantly increases GDM incidence with this approach 1, 2, 7

Alternative Criteria (NDDG)

The older National Diabetes Data Group criteria use slightly higher thresholds for the 100g OGTT: fasting ≥105 mg/dL, 1h ≥190 mg/dL, 2h ≥165 mg/dL, 3h ≥145 mg/dL 1

Early Pregnancy Overt Diabetes Diagnosis

If testing at the first prenatal visit reveals fasting plasma glucose ≥126 mg/dL or random glucose ≥200 mg/dL (confirmed on a subsequent day), diagnose overt diabetes rather than GDM 1, 3

Key Clinical Considerations

• The one-step approach requires all women to fast, making it more burdensome but eliminates the need for a second test 1, 6
• The two-step approach does not require fasting for initial screening, improving patient convenience and compliance 1
• Both approaches are acceptable, and the choice depends on your practice setting, patient population prevalence, and available resources 1, 6
• Do not perform OGTT after bariatric surgery due to risk of postprandial hypoglycemia; use alternative monitoring strategies 3

Postpartum Follow-Up

Test all women with GDM at 4-12 weeks postpartum using a 75g OGTT with non-pregnancy diagnostic criteria to identify persistent diabetes or prediabetes 1, 2, 7

Continue lifelong screening at least every 3 years, as women with prior GDM have a 3.4-fold increased risk of developing type 2 diabetes 1, 2, 7