What is the best approach to screen for gestational diabetes mellitus (GDM) in a 23-week pregnant woman with a family history of diabetes, considering options such as A1c (A1c), 75g oral glucose tolerance test (OGTT) at 24-28 weeks, or a 3-hour OGTT at 24-28 weeks?

Screening for Gestational Diabetes in a 23-Week Pregnant Woman with Family History of Diabetes

You should perform a 75g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation (Option B), as this patient has a strong family history of diabetes making her high-risk, and current guidelines recommend standard screening during this window regardless of approach chosen. 1, 2

Risk Stratification and Timing

This patient qualifies as high-risk due to her family history of diabetes, which warrants careful screening consideration. 1, 2

• High-risk patients with marked obesity, personal history of GDM, glycosuria, or strong family history of diabetes should undergo glucose testing as soon as feasible in pregnancy, and if negative, be retested between 24-28 weeks. 1
• Since she is already at 23 weeks and presenting now, the optimal timing is to proceed with screening at 24-28 weeks of gestation, which is the standard evidence-based window. 1, 2

Why NOT HbA1c (Option A)

HbA1c is not recommended for diagnosing gestational diabetes mellitus (GDM). 1

• The IADPSG GDM diagnostic criteria for the 75g OGTT were not derived from data using HbA1c and should not be used for GDM screening. 1
• HbA1c has insufficient sensitivity and specificity for GDM diagnosis, with research showing it cannot replace OGTT. 3
• HbA1c is unreliable at 15 weeks of gestation and beyond for GDM screening due to inaccuracies from increased red blood cell turnover in pregnancy. 1
• HbA1c may be useful only for detecting pre-existing diabetes in early pregnancy (using non-pregnant thresholds ≥6.5%), but not for GDM diagnosis. 1

Choosing Between 75g OGTT (Option B) vs 3-Hour 100g OGTT (Option C)

Both are acceptable approaches, but they represent different diagnostic strategies:

One-Step Approach: 75g OGTT (Option B - Recommended)

• Perform a 75g OGTT with measurements at fasting, 1-hour, and 2-hours at 24-28 weeks. 1, 4, 2
• Diagnosis requires only ONE elevated value: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL. 1, 4, 2
• This approach is supported by the International Association of Diabetes and Pregnancy Study Groups (IADPSG) and American Diabetes Association. 1, 4
• Requires fasting for at least 8 hours before the test. 1, 4
• Identifies approximately 15-20% of pregnant women with GDM. 1, 5

Two-Step Approach: 3-Hour 100g OGTT (Option C - Alternative)

• Step 1: Initial 50g glucose load test (non-fasting) with 1-hour measurement. 1, 2
• Step 2: If screening threshold exceeded (≥130,135, or 140 mg/dL), proceed to 100g OGTT with measurements at fasting, 1-hour, 2-hours, and 3-hours. 1, 2
• Diagnosis requires at least TWO elevated values: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, or 3-hour ≥140 mg/dL (Carpenter-Coustan criteria). 1, 4, 2
• This approach is currently supported by the American College of Obstetricians and Gynecologists (ACOG). 1, 2
• Identifies approximately 5-6% of pregnant women with GDM. 1, 5
• Note: ACOG acknowledges that one elevated value can be used for diagnosis, though traditionally two values are required. 1

Clinical Decision Algorithm

For this patient at 23 weeks with family history:

1. Schedule testing at 24-28 weeks (she's in the ideal window). 1, 2
2. Choose the 75g OGTT (one-step approach) as it is simpler, requires only one visit, identifies more cases of GDM (2-3 times more than two-step), and is cost-effective in high-risk populations. 1, 4, 5
3. The patient must fast for at least 8 hours before the test. 1, 4
4. If any single value meets or exceeds thresholds, diagnose GDM and initiate management. 1, 4, 2

Important Caveats

• The debate continues: The one-step approach identifies more women with GDM but without clear evidence of superior pregnancy outcomes in all studies, leading to ongoing controversy about resource utilization. 5
• Local practice patterns matter: Some institutions prefer the two-step approach due to ACOG support and lower diagnosis rates. 1, 2, 5
• For high-risk patients like this one, the one-step approach may be more cost-effective and identifies women who would benefit from intervention. 1

Post-Diagnosis Management

If GDM is diagnosed:

• Begin lifestyle modification immediately (medical nutrition therapy and exercise). 6
• Target glucose levels: fasting <95 mg/dL, 1-hour postprandial <140 mg/dL, 2-hour postprandial <120 mg/dL. 6
• Add insulin therapy if targets not achieved within 1-2 weeks of lifestyle modification. 6
• Screen for persistent diabetes at 4-12 weeks postpartum using 75g OGTT. 1, 2, 6