What are the diagnostic criteria and management strategies for Gestational Diabetes Mellitus (GDM)?

Gestational Diabetes Mellitus: Diagnosis and Assessment of Control

Diagnostic Approach

Either the one-step or two-step diagnostic strategy is acceptable for GDM screening at 24-28 weeks of gestation, though the one-step approach identifies approximately twice as many cases and is the only method based on pregnancy outcomes rather than prediction of maternal diabetes. 1, 2

One-Step Strategy (IADPSG Criteria)

Perform a 75-g oral glucose tolerance test (OGTT) with plasma glucose measurements at fasting, 1-hour, and 2-hour after an overnight fast of at least 8 hours. 1

GDM is diagnosed when ANY single value meets or exceeds: 1, 2

• 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)

This approach identifies 15-20% of pregnant women with GDM and is based on the landmark HAPO study demonstrating continuous risk of adverse outcomes without a clear threshold. 1, 2

Two-Step Strategy (Carpenter-Coustan Criteria)

Step 1: Perform a 50-g glucose load test (GLT) without fasting, with plasma glucose measurement at 1 hour. 1

• If ≥130,135, or 140 mg/dL (commonly 140 mg/dL), proceed to Step 2. 1

Step 2: Perform a 100-g OGTT after overnight fast with measurements at fasting, 1-hour, 2-hour, and 3-hour. 1

GDM is diagnosed when at least TWO of the following are met or exceeded: 1, 2

• 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 a single elevated value may be used for diagnosis. 1, 2

This approach identifies 5-6% of pregnant women with GDM and is easier to implement as the initial screening does not require fasting. 2

Early Screening for High-Risk Women

Test at the first prenatal visit using standard diagnostic criteria (fasting glucose ≥126 mg/dL or random glucose ≥200 mg/dL) in women with: 1

• Marked obesity
• Personal history of GDM
• Strong family history of diabetes
• Glycosuria (though note: glycosuria alone is unreliable due to decreased renal threshold in pregnancy) 3

If negative, retest at 24-28 weeks using standard GDM screening protocols. 1, 4

Assessment of Glycemic Control

Self-Monitoring Protocol

Women with GDM must perform self-monitoring of blood glucose (SMBG) at least 4 times daily: 1, 3

• Fasting (upon waking)
• 1-hour OR 2-hour postprandial after each meal (breakfast, lunch, dinner)

Target Glucose Values

The following targets must be achieved to avoid pharmacologic therapy: 1, 5, 3

• Fasting plasma glucose: <95 mg/dL (<5.3 mmol/L)
• 1-hour postprandial: <140 mg/dL (<7.8 mmol/L) OR
• 2-hour postprandial: <120 mg/dL (<6.7 mmol/L)

Treatment Algorithm

Initiate lifestyle modification immediately upon diagnosis (medical nutrition therapy with trained dietician and regular physical activity). 5, 3

Document glucose patterns over 1-2 weeks. 3

If glycemic targets are not achieved within 1-2 weeks of lifestyle modification, add insulin therapy. 5, 3

• Insulin is the preferred first-line pharmacologic agent during pregnancy. 5, 3
• Approximately 70-85% of women achieve adequate control with lifestyle modification alone. 5, 3

Fetal Surveillance

Perform ultrasound surveillance to assess fetal abdominal circumference. 5, 3

• Measurements exceeding the 75th percentile for gestational age indicate fetal hyperinsulinism and require intensification of glycemic control. 5, 3

Critical Monitoring Pitfall

Monitor for rapid reduction in insulin requirements in late pregnancy, as this may indicate placental insufficiency requiring immediate evaluation. 5

Postpartum Management

Screen for persistent diabetes at 4-12 weeks postpartum using a 75-g OGTT with standard non-pregnancy diagnostic criteria. 1, 5

Counsel all women with GDM about their 3.4-fold increased risk of developing type 2 diabetes and the need for lifelong screening at least every 3 years. 1, 5, 3

Women found to have prediabetes postpartum should receive intensive lifestyle interventions or metformin to prevent progression to diabetes. 1, 3

Important Clinical Caveats

• Urine glucose testing is unreliable during pregnancy due to decreased renal threshold and should not guide treatment decisions. 3
• The one-step approach identifies more women with GDM but requires all women to fast, while the two-step approach is more convenient for initial screening. 2
• A recent randomized trial found no difference in pregnancy outcomes between one-step and two-step approaches despite treating twice as many women with the one-step method, though methodological concerns were raised. 1
• HbA1c is not recommended for GDM diagnosis or monitoring during pregnancy. 1