The 75-gram, 2-hour oral glucose tolerance test (OGTT) is recommended at 24–28 weeks gestation for patients not previously diagnosed with overt diabetes or gestational diabetes mellitus (GDM).
| Units | Fasting | 1 Hour | 2 Hour |
|---|---|---|---|
| mg/dL | 92 | 180 | 153 |
| mmol/L | 5.1 | 10 | 8.5 |
Diagnosis of GDM: GDM is diagnosed when any one value meets or exceeds the threshold using the IADPSG criteria.
Overt diabetes: Fasting glucose ≥ 126 mg/dL (7.0 mmol/L).
Testing Requirements
- Overnight fast of 8–14 hours
- ≥3 days of unrestricted diet (≥150 g carbohydrate/day)
- No smoking; remain seated during test
Causes of High Fasting Glucose
- Pregnancy-related insulin resistance
- Preexisting type 2 diabetes
- Gestational diabetes
- Glucocorticoid use
- Increased hepatic glucose production
- Cushing syndrome
- Sleep apnea
Causes of High 1-Hour Glucose
- Impaired first-phase insulin response
- Rapid gastric emptying
- Placental hormonal insulin resistance
- High carbohydrate intake before testing
- Obesity
Causes of High 2-Hour Glucose
- Gestational diabetes (most common)
- Impaired glucose tolerance
- Reduced second-phase insulin secretion
- Medications:
- Glucocorticoids
- Beta-agonists
- Physical inactivity
Causes of Low OGTT Values (Hypoglycemia)
- Reactive hypoglycemia
- Dumping syndrome (post-bariatric surgery)
- Insulinoma (rare)
- Prolonged fasting
- Insufficient glucose ingestion
- Recent strenuous exercise
References
- International Association of Diabetes and Pregnancy Study Groups (IADPSG) Consensus Panel. Diabetes Care. 2010;33:676–82. PMID: 20190296.