What is the recommended screening protocol for gestational diabetes mellitus, including timing, glucose challenge test thresholds, diagnostic oral glucose tolerance test criteria, and early screening indications for high‑risk pregnant women?

Screening for Gestational Diabetes Mellitus

All pregnant women without pre-existing diabetes should undergo universal screening for gestational diabetes at 24–28 weeks of gestation using either a one-step 75-g oral glucose tolerance test or a two-step approach beginning with a 50-g glucose challenge test. 1

Risk Assessment and Early Screening (First Prenatal Visit, ~12–14 Weeks)

High-risk women require glucose testing at the first prenatal visit to detect pre-existing undiagnosed type 2 diabetes, and if negative, must be retested at 24–28 weeks. 2, 1

High-Risk Criteria Warranting Early Screening:

• BMI ≥30 kg/m² (the single most important risk factor) 1
• Personal history of gestational diabetes 2, 1
• Glycosuria detected on routine urinalysis 2
• First-degree relative with diabetes 1
• High-risk ethnicity (Hispanic, Native American, South or East Asian, African American, Pacific Islander) 1
• History of delivering a macrosomic infant (>4.05 kg or 9 lb) 1
• History of polycystic ovary syndrome 1

Early Pregnancy Diagnostic Thresholds (Indicating Overt Pre-Existing Diabetes):

• Fasting plasma glucose ≥126 mg/dL 1, 3
• Random plasma glucose ≥200 mg/dL with hyperglycemia symptoms 1, 3
• HbA1c ≥6.5% (though not recommended for routine GDM screening) 1, 3

Critical pitfall: Failing to repeat screening at 24–28 weeks in high-risk women who initially test negative is the most common error, as insulin resistance increases exponentially in the second and third trimesters. 1

Low-Risk Women Who May Skip Screening

Women meeting all of the following criteria may forgo glucose testing entirely: 2, 1

• Age <25 years 2, 1
• Pre-pregnancy BMI ≤25 kg/m² 2, 1
• No first-degree relative with diabetes 2, 1
• No history of abnormal glucose tolerance 2, 1
• No history of adverse obstetric outcomes 1
• Belonging to an ethnic group with low diabetes prevalence 2, 1

In practice, most pregnant women require screening because these exemption criteria are stringent. 1

Standard Universal Screening at 24–28 Weeks

One-Step Approach (IADPSG/ADA Criteria)

Perform a single 75-g oral glucose tolerance test with plasma glucose measured fasting, at 1 hour, and at 2 hours. 1, 4

Diagnostic thresholds—GDM is diagnosed if ANY ONE value is met or exceeded: 1, 4

• Fasting ≥92 mg/dL (5.1 mmol/L) 1, 4
• 1-hour ≥180 mg/dL (10.0 mmol/L) 1, 4
• 2-hour ≥153 mg/dL (8.5 mmol/L) 1, 4

These thresholds are derived from the HAPO study, which demonstrated a continuous relationship between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) without a clear risk threshold. 1, 4

The one-step approach identifies approximately 15–20% of pregnancies with GDM and is more cost-effective in high-risk populations. 4

Two-Step Approach (ACOG-Supported)

Step 1: Administer a 50-g glucose challenge test (non-fasting) at 24–28 weeks. 2, 1

• If 1-hour plasma glucose is ≥130–140 mg/dL, proceed to Step 2 2, 1
• The 140 mg/dL threshold offers higher specificity while maintaining 100% sensitivity 5

Step 2: Perform a 100-g oral glucose tolerance test (fasting) with measurements at fasting, 1,2, and 3 hours. 2, 1

Diagnostic thresholds (Carpenter-Coustan criteria)—GDM is diagnosed if AT LEAST TWO values are met or exceeded: 2, 4

• Fasting ≥95 mg/dL (5.3 mmol/L) 2, 4
• 1-hour ≥180 mg/dL (10.0 mmol/L) 2, 4
• 2-hour ≥155 mg/dL (8.6 mmol/L) 2, 4
• 3-hour ≥140 mg/dL (7.8 mmol/L) 2, 4

Note: ACOG acknowledges that in clinical practice, a single elevated value may be used for diagnosis, though traditional criteria require two abnormal values. 4

The two-step approach identifies approximately 5–6% of pregnancies with GDM but may miss milder cases that still benefit from treatment. 4

Test Preparation Requirements

Both approaches require specific preparation to ensure accurate results: 2, 4

• 8–14 hour overnight fast before the OGTT 2, 4
• Consume ≥150 g carbohydrate daily for at least 3 days before testing 2, 4
• Maintain unrestricted physical activity in the days preceding the test 2
• Patient must remain seated throughout the test and not smoke 2, 4

Postpartum and Long-Term Follow-Up

All women diagnosed with GDM must undergo a 75-g OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes. 1, 6

Lifelong diabetes screening is mandatory at least every 3 years for women with prior GDM, who have a 3.4-fold increased risk of developing type 2 diabetes. 1, 6

Women with postpartum prediabetes should receive intensive lifestyle interventions or metformin therapy to prevent progression to overt diabetes. 1, 6

Key Clinical Caveats

• Do not use HbA1c for GDM screening or diagnosis—it has limited sensitivity for detecting glucose intolerance during pregnancy. 7, 8
• Do not rely on urine glucose testing—it is not useful for GDM management. 4
• Do not postpone screening beyond 28 weeks—the 24–28 week window aligns with peak insulin resistance and enables timely intervention. 1
• Do not apply GDM diagnostic thresholds (92/180/153 mg/dL) to early pregnancy screening—use overt diabetes criteria (fasting ≥126 mg/dL) instead. 1
• After bariatric surgery, do not perform an OGTT due to risk of postprandial hypoglycemia. 3, 6