What are the recommended screening, diagnostic criteria, and management strategies for gestational diabetes in pregnant women?

Gestational Diabetes: Screening, Diagnosis, and Management

Screening Strategy

All pregnant women without known diabetes should undergo universal screening for gestational diabetes at 24-28 weeks of gestation, with high-risk women requiring additional early screening at their first prenatal visit. 1, 2, 3

Early Screening (First Prenatal Visit)

Screen immediately at the first prenatal visit if any of the following high-risk factors are present: 1, 2, 4

• BMI ≥30 kg/m² (most important risk factor) 2, 4
• History of previous gestational diabetes 2, 3
• First-degree relative with diabetes 2
• High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 2
• Previous macrosomic infant (>4.5 kg) 2
• Polycystic ovary syndrome 2
• Glycosuria on routine testing 2

Critical point: If early screening is negative in high-risk women, mandatory repeat screening at 24-28 weeks is required because insulin resistance increases exponentially in the second and third trimesters. 2, 4

Low-Risk Exemption Criteria

Women may skip screening only if they meet ALL of the following criteria (rarely applicable in practice): 2, 3

• Age <25 years
• Pre-pregnancy BMI ≤25 kg/m²
• No first-degree relative with diabetes
• No history of abnormal glucose metabolism
• Low-risk ethnic group

Diagnostic Approaches

Two acceptable strategies exist, with ongoing debate about which is optimal: 1, 3

Two-Step Approach (Preferred by ACOG)

Step 1: 50-g glucose challenge test (non-fasting) at 24-28 weeks 1, 2, 3

• Plasma glucose measured at 1 hour
• Threshold: ≥130-140 mg/dL (institutional variation) triggers Step 2 1

Step 2: 100-g oral glucose tolerance test (fasting, after overnight fast of 8+ hours) 1, 3

Diagnosis requires ≥2 abnormal values (Carpenter-Coustan criteria): 1, 3

• Fasting: ≥95 mg/dL (5.3 mmol/L)
• 1 hour: ≥180 mg/dL (10.0 mmol/L)
• 2 hours: ≥155 mg/dL (8.6 mmol/L)
• 3 hours: ≥140 mg/dL (7.8 mmol/L)

One-Step Approach (IADPSG/International Consensus)

Single 75-g oral glucose tolerance test (fasting) at 24-28 weeks 1, 2, 3

Diagnosis requires only 1 abnormal value: 1, 2, 3

• Fasting: ≥92 mg/dL (5.1 mmol/L)
• 1 hour: ≥180 mg/dL (10.0 mmol/L)
• 2 hours: ≥153 mg/dL (8.5 mmol/L)

Important nuance: The one-step approach identifies approximately twice as many women with GDM compared to the two-step method, based on the HAPO study showing continuous risk without clear thresholds. 1, 3 The two-step approach remains more commonly used in U.S. practice. 2, 3

Early Pregnancy Detection of Pre-Existing Diabetes

At the first prenatal visit in high-risk women, the following thresholds indicate overt diabetes (not gestational diabetes) requiring immediate treatment: 1, 4, 3

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L)
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms
• HbA1c ≥6.5% (though not recommended for routine GDM screening at 24-28 weeks) 4, 3

Management Targets

Blood glucose targets for women diagnosed with GDM: 3

• Fasting: <95 mg/dL
• 1 hour postprandial: <140 mg/dL
• 2 hours postprandial: <120 mg/dL

Treatment hierarchy: 1, 3

1. Nutritional counseling and lifestyle modifications (first-line)
2. Self-monitoring of blood glucose
3. Moderate-intensity physical activity (if not contraindicated)
4. Insulin therapy if targets not achieved (first-choice medication)
5. Metformin as alternative (though insulin remains preferred)

Postpartum and Long-Term Follow-Up

Immediate postpartum (4-12 weeks after delivery): 1, 2, 4, 3

• Mandatory 75-g oral glucose tolerance test using non-pregnancy diagnostic criteria to identify persistent diabetes or prediabetes
• Do not use HbA1c for this postpartum screening 4

Lifelong surveillance: 1, 2, 3

• Screen for diabetes or prediabetes at least every 3 years throughout life
• Women with history of GDM have 3.4-7 times higher risk of developing type 2 diabetes 3

If prediabetes detected: 1, 2

• Intensive lifestyle interventions (weight management, physical activity, healthy diet)
• Metformin therapy to prevent progression to type 2 diabetes

Critical Pitfalls to Avoid

• Failing to rescreen at 24-28 weeks in high-risk women with negative early screening—this is the most common error, as GDM typically develops later in pregnancy 2, 4
• Using HbA1c for routine GDM screening at 24-28 weeks—glucose tolerance testing is superior 4
• Assuming low-risk status without verifying ALL exemption criteria are met—most pregnant women require screening 2, 3
• Neglecting postpartum follow-up—many women are lost to follow-up despite high diabetes risk 1, 3
• Not counseling about future pregnancy risk—women with prior GDM should undergo preconception screening before subsequent pregnancies 2, 3

Evidence Quality Note

The diagnostic criteria are derived from the landmark HAPO study, which demonstrated continuous associations between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) without clear thresholds. 1, 3 Treatment of GDM reduces serious perinatal complications with a number needed to treat of 34. 2