What is the most appropriate initial screening test for gestational diabetes in a 22-week pregnant woman with a family history of Diabetes Mellitus (DM)?

Screening for Gestational Diabetes in a 22-Week Pregnant Woman with Family History of Diabetes

This patient should undergo a 75g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation, which is the standard screening window for all pregnant women, including those with a family history of diabetes. 1, 2

Risk Assessment and Timing

This 22-week pregnant woman with a family history of diabetes (mother with DM) falls into the average-risk category for gestational diabetes mellitus (GDM), not high-risk, based on current guidelines. 1

High-risk criteria requiring immediate screening include:

• Marked obesity (BMI ≥30 kg/m²)
• Personal history of previous GDM
• Glycosuria detected on urinalysis
• Strong family history of diabetes (first-degree relative)

1, 2

Since BMI was not mentioned and she has no other high-risk features beyond family history, she does not meet criteria for early screening at 22 weeks. 2

Recommended Screening Approach

At 24-28 weeks gestation, perform one of the following:

Option 1: One-Step Approach (75g OGTT) - Preferred

• Perform a 75g OGTT with fasting, 1-hour, and 2-hour glucose measurements
• Diagnostic thresholds (any ONE value diagnostic):
  • 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)

1, 2

Option 2: Two-Step Approach (Alternative)

• Step 1: 50g glucose challenge test (non-fasting) with 1-hour glucose measurement
• Step 2: If ≥140 mg/dL (or ≥130-135 mg/dL in high-risk populations), proceed to 100g OGTT
• 100g OGTT diagnostic thresholds (TWO or more values required):
  • 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)

1, 2, 3

Why NOT the Other Options

Fasting glucose now (at 22 weeks): Not recommended as a standalone screening test at this gestational age. Fasting glucose is only used for early screening in high-risk women at first prenatal visit (12-14 weeks), and this patient doesn't meet high-risk criteria. 1, 2

HbA1c now: Not recommended for GDM screening during pregnancy. HbA1c is only useful for detecting pre-existing overt diabetes in early pregnancy (before 20 weeks) in high-risk women, not for GDM screening. 1, 4

The 3-hour OGTT at 24-28 weeks: This is the 100g OGTT used in the two-step approach, but it's only performed AFTER a positive 50g glucose challenge test, not as the initial screening test. 1, 3

Important Clinical Considerations

The 75g OGTT (one-step approach) has several advantages:

• Faster diagnosis with single test
• Better patient tolerance
• Outcome-based criteria from the HAPO study
• Simpler to execute and remember
• More aligned with international consensus

1, 5, 6

Common pitfall to avoid: Do not perform early screening (before 24 weeks) in average-risk women, as there is insufficient evidence for benefit and it may lead to overdiagnosis and unnecessary interventions. 7, 3

If this patient had BMI ≥30 kg/m² or previous GDM: She would require screening at first prenatal visit AND repeat screening at 24-28 weeks if initially negative. 2