Diagnosis of Gestational Diabetes in mmol/L
The diagnosis of gestational diabetes requires a 75g oral glucose tolerance test (OGTT) with gestational diabetes diagnosed if any single value meets or exceeds: fasting ≥5.1 mmol/L, 1-hour ≥10.0 mmol/L, or 2-hour ≥8.5 mmol/L. 1, 2, 3
Diagnostic Thresholds Based on IADPSG Criteria
The International Association of Diabetes and Pregnancy Study Groups (IADPSG) established these thresholds based on the landmark HAPO study, which demonstrated continuous associations between maternal glucose levels and adverse pregnancy outcomes including macrosomia, preeclampsia, and cesarean delivery 4, 5. These criteria represent the only outcome-based diagnostic thresholds for gestational diabetes 5, 6.
The specific diagnostic values are:
- Fasting plasma glucose ≥5.1 mmol/L (92 mg/dL) 4, 1, 2
- 1-hour post-load ≥10.0 mmol/L (180 mg/dL) 4, 1, 2
- 2-hour post-load ≥8.5 mmol/L (153 mg/dL) 4, 1, 2, 3
Critical Distinction: Early Pregnancy vs. Standard Screening
Early Pregnancy (First Prenatal Visit)
For high-risk women screened at the first prenatal visit, the interpretation differs:
- Fasting glucose <5.1 mmol/L: Normal, but mandatory repeat screening at 24-28 weeks 1, 2
- Fasting glucose 5.1-6.9 mmol/L: Early gestational diabetes requiring immediate management 1
- Fasting glucose ≥7.0 mmol/L: Overt pre-existing diabetes, not gestational diabetes 1
Standard Screening (24-28 Weeks)
Universal screening occurs at 24-28 weeks of gestation using the 75g OGTT with the thresholds listed above 1, 2. This timing is optimal because the HAPO study demonstrated that glucose levels at this gestational age have the strongest continuous associations with adverse outcomes 2.
One-Step vs. Two-Step Approach
The American Diabetes Association recommends the one-step approach using the 75g OGTT with diagnosis made if any single value is abnormal 1, 2. This represents a paradigm shift from requiring multiple abnormal values 3.
The American College of Obstetricians and Gynecologists alternatively supports a two-step approach: initial 50g glucose challenge test (non-fasting) followed by diagnostic 100g OGTT if the challenge test shows glucose ≥7.8 mmol/L (140 mg/dL) 2, 6. However, the screening thresholds for the 50g test are:
- ≥7.21 mmol/L (130 mg/dL): 99% sensitivity, 77% specificity 4
- ≥7.77 mmol/L (140 mg/dL): 85% sensitivity, 86% specificity 4
Fasting Plasma Glucose Alone for Screening
While fasting plasma glucose can be used for screening, it has variable test characteristics depending on the threshold 4:
- ≥4.72 mmol/L (85 mg/dL): 87% sensitivity, 52% specificity
- ≥5.00 mmol/L (90 mg/dL): 77% sensitivity, 76% specificity
- ≥5.11 mmol/L (92 mg/dL): 76% sensitivity, 92% specificity
- ≥5.27 mmol/L (95 mg/dL): 54% sensitivity, 93% specificity
The OGTT remains superior because each of the three time points (fasting, 1-hour, 2-hour) contributes independently to predicting adverse outcomes 4.
Common Pitfalls to Avoid
Do not skip repeat screening at 24-28 weeks in high-risk women with normal early pregnancy glucose values, as gestational diabetes typically manifests in the late second trimester due to increasing insulin resistance 2.
Do not use HbA1c for diagnosis of gestational diabetes, as it has poor test characteristics (sensitivity 82%, specificity 21% at 5.5% threshold) compared to OGTT 4.
Do not delay treatment when fasting glucose is ≥5.1 mmol/L at any time during pregnancy in high-risk women, as treatment at these mild glucose elevations reduces preeclampsia, macrosomia, and shoulder dystocia 4, 1.
Treatment Targets After Diagnosis
Once diagnosed, the American Diabetes Association recommends the following glucose targets 1:
- Fasting <5.3 mmol/L (95 mg/dL)
- 1-hour postprandial <7.8 mmol/L (140 mg/dL)
- 2-hour postprandial <6.7 mmol/L (120 mg/dL)
- HbA1c <6% (42 mmol/mol) if achieved without significant hypoglycemia