Elevated 3-Hour Glucose Tolerance Test at 28 Weeks Indicates Gestational Diabetes Mellitus
An elevated value on the 3-hour glucose tolerance test at 28 weeks of pregnancy indicates a diagnosis of gestational diabetes mellitus (GDM), which requires immediate initiation of treatment to reduce risks of adverse pregnancy outcomes including macrosomia, cesarean delivery, neonatal hypoglycemia, and long-term metabolic complications for both mother and child. 1
Diagnostic Criteria for the 3-Hour Test
The 3-hour, 100-gram oral glucose tolerance test (OGTT) is part of the two-step diagnostic approach for GDM. GDM is diagnosed when two or more of the following venous plasma glucose thresholds are met or exceeded 1:
- 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)
The American College of Obstetricians and Gynecologists notes that even one elevated value can be used for diagnosis, though the traditional Carpenter-Coustan criteria require two or more abnormal values 1, 2.
Clinical Significance and Immediate Risks
An elevated 3-hour glucose tolerance test identifies maternal hyperglycemia that has graded, continuous associations with adverse pregnancy outcomes 1. The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study demonstrated strong linear relationships between maternal glucose levels and:
- Large for gestational age infants (birth weight >90th percentile) 1
- Cesarean delivery 1
- Neonatal hypoglycemia requiring clinical intervention 1
- Elevated cord serum C-peptide (>90th percentile), indicating fetal hyperinsulinemia 1
- Increased neonatal adiposity 1
Immediate Management Requirements
Once GDM is diagnosed, strict metabolic control is mandatory 3, 4. The management algorithm proceeds as follows:
First-Line Interventions (All Patients)
- Nutritional counseling with carbohydrate restriction (≥150 g/day during testing preparation) 1
- Blood glucose self-monitoring instruction 3, 4
- Moderate-intensity physical activity unless contraindicated 3, 4
Glycemic Targets
Insulin therapy should be initiated when medical nutrition therapy fails to maintain 1:
- Fasting plasma glucose <95 mg/dL (5.3 mmol/L)
- 1-hour postprandial <140 mg/dL (7.8 mmol/L)
- 2-hour postprandial <130 mg/dL (7.2 mmol/L)
Insulin is the first-choice pharmacologic therapy when lifestyle modifications are insufficient 3, 4.
Maternal and Fetal Monitoring
- Regular obstetric examinations with ultrasound to assess fetal growth, particularly abdominal circumference in the third trimester 1, 4
- Neonatal blood glucose monitoring after delivery due to risk of hypoglycemia 3, 4
Long-Term Implications
Maternal Risk
Women diagnosed with GDM face substantially elevated long-term metabolic risk 1:
- Sevenfold increased relative risk of developing type 2 diabetes 3
- 3.4-fold higher risk of prediabetes and type 2 diabetes within 11 years postpartum 1, 2, 5
- Increased cardiovascular disease risk 4
All women with GDM must undergo re-evaluation with a 75-gram OGTT at 4-12 weeks postpartum using standard WHO criteria 5, 3, 4. If normal glucose tolerance is confirmed, lifelong screening for diabetes or prediabetes is required at least every 2-3 years 5, 4.
Offspring Risk
Children born to mothers with GDM have increased risk of 1:
- Childhood obesity
- Glucose intolerance and diabetes in late adolescence and young adulthood
- Increased body fat percentage
Critical Pitfalls to Avoid
Do not delay treatment initiation while awaiting additional testing—the diagnosis is established once criteria are met, and immediate intervention reduces adverse outcomes 1, 3.
Do not assume normal postpartum glucose tolerance—approximately 31-34% of women develop impaired glucose tolerance and 3% develop overt diabetes within weeks of delivery 6, 4.
Do not discontinue monitoring after delivery—the diagnosis of GDM confers lifelong metabolic risk requiring ongoing surveillance and preventive interventions including weight management and physical activity 5, 3, 4.