GDM is diagnosed if ANY ONE of the thresholds above is met or exceeded.
One-step 75g OGTT is performed at 24–28 weeks of gestation in women not previously diagnosed with overt diabetes.
Overt (Pre-existing) Diabetes: Fasting ≥ 7.0 mmol/L, HbA1c ≥ 6.5%, or Random Glucose ≥ 11.1 mmol/L.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
Primary Clinical Uses
Universal screening for Gestational Diabetes Mellitus (GDM) between 24–28 weeks of gestation.
Early screening in the first trimester for high-risk patients (BMI > 30, history of GDM, strong family history) to identify overt pre-existing T2DM.
The Great Debate
There is a massive global divide between the "One-Step" approach (endorsed by WHO / IADPSG / ADA) and the "Two-Step" approach (endorsed by ACOG and NIH in the United States).
Section 2
Formula & Logic
One-Step Method (WHO / IADPSG)
Administer a 75g Oral Glucose Tolerance Test (OGTT) while fasting.
Diagnosis is made if AT LEAST ONE value is elevated:
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 Method (ACOG / NIH)
Step 1: Non-fasting 50g Glucose Challenge Test (GLT). If 1-hour value is ≥ 130, 135, or 140 mg/dL (institution dependent), proceed to Step 2.
The 75g One-Step criteria drastically increases GDM prevalence to ~18% of all pregnancies.
ACOG argues this heavily medicalizes borderline pregnancies, increasing anxiety and healthcare costs WITHOUT strongly proven reductions in clear morbidity (e.g., shoulder dystocia, NICU admissions).
If a patient vomits the 100g glucose load during the 3-hour test (very common), the test is invalid. Alternative strategies like fasting blood sugar + random postprandial tracking or immediate progression to a 75g test may be necessary depending on institutional policy.
Section 4
Next Steps
First-Line Management Strategies
01
Initiate Medical Nutrition Therapy (MNT): Caloric restriction tailored to BMI, substituting high glycemic index carbs with complex carbs/protein.
02
Prescribe glucometer: Track fasting (goal < 95 mg/dL) and either 1-hr postprandial (< 140 mg/dL) or 2-hr postprandial (< 120 mg/dL).
03
If >20% of values over 1-2 weeks cross thresholds: Initiate pharmacotherapy.
04
Insulin is the absolute gold standard because it does not cross the placenta. Metformin is used extensively but DOES cross the placenta (long-term metabolic effects on offspring are under intense study).
Section 5
Evidence Appraisal
The HAPO Study
Hyperglycemia and adverse pregnancy outcomes.
HAPO Study Cooperative Research Group. • N Engl J Med.. 2008;The landmark observational study (n=23,000+) proving a continuous, graded relationship between maternal glucose levels and adverse outcomes (c-section, birth weight >90th percentile) even BELOW diagnostic diabetic thresholds. This directly birthed the IADPSG One-Step criteria.
Section 6
Literature
O'Sullivan and Mahan
The original 100g 3-hour test thresholds were established in 1964 by O'Sullivan and Mahan, based solely on predicting which mothers would develop Type 2 Diabetes later in life, entirely ignoring fetal outcomes. Carpenter and Coustan later mathematically lowered these thresholds in 1982 to match modern plasma-based glucose testing.
