In a 23‑week pregnant woman with a mother who has type 2 diabetes mellitus, which test is appropriate for gestational diabetes mellitus screening?

Gestational Diabetes Screening at 23 Weeks Gestation

This patient should undergo a 75g oral glucose tolerance test (OGTT) at 24-28 weeks gestation (option C), which is the standard universal screening window for all pregnant women without previously diagnosed diabetes. 1, 2

Why Standard Screening at 24-28 Weeks is Appropriate

Universal screening between 24-28 weeks is the evidence-based standard for all pregnant women, regardless of risk factors, because this timing corresponds to the period of maximal insulin resistance when gestational diabetes most commonly manifests. 1, 2

• The American Diabetes Association, American College of Obstetricians and Gynecologists, and U.S. Preventive Services Task Force all recommend screening at 24-28 weeks gestation for women not previously known to have diabetes. 1
• This recommendation carries a B-level grade from the USPSTF, indicating moderate net benefit for reducing preeclampsia, fetal macrosomia, and shoulder dystocia. 1
• The HAPO study demonstrated a continuous, graded relationship between maternal glucose levels at 24-28 weeks and adverse outcomes including macrosomia, cesarean delivery, neonatal hypoglycemia, and shoulder dystocia. 2

Why This Patient Does NOT Need Early Screening

A family history of type 2 diabetes in the mother (second-degree relative) alone does NOT meet high-risk criteria for early screening at 23 weeks. 2

High-risk criteria requiring immediate first-trimester screening include:

• BMI ≥30 kg/m² (the single most important risk factor) 2, 3
• Personal history of previous GDM 1, 2
• First-degree relative with diabetes (not second-degree) 2
• Glycosuria on routine urinalysis 2
• High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 2

Since this patient has uncomplicated previous pregnancies and only a maternal family history (which is common and does not constitute high-risk), she should follow the standard screening protocol. 2

Why the Other Options Are Incorrect

Option A: Hemoglobin A1C at This Visit

• A1C is NOT recommended for GDM screening because it has poor sensitivity and specificity for detecting gestational diabetes. 1, 3
• A1C may be useful for detecting pre-existing type 2 diabetes in early pregnancy but is not the appropriate test for GDM screening. 1

Option B: Random Blood Sugar at This Visit

• Random glucose measurements are not validated or recommended for GDM screening. 1
• Random glucose ≥200 mg/dL with hyperglycemic symptoms would indicate overt diabetes, but this is not a screening test. 2

Option D: 3-Hour Glucose Tolerance Test at 28-32 Weeks

• The timing is suboptimal—screening should occur at 24-28 weeks, not 28-32 weeks. 1, 2
• The 3-hour 100g OGTT is part of the two-step approach (used after a positive 50g glucose challenge test), not a primary screening test. 1
• The 75g OGTT is the internationally recognized one-step approach and is simpler, faster, and better tolerated than the two-step method. 2, 4

Recommended Testing Protocol

At 24-28 weeks gestation, perform a 75g OGTT with the following procedure:

• Require an 8-14 hour overnight fast. 2
• Ensure at least 3 days of unrestricted diet with ≥150g carbohydrate daily before testing. 2, 3
• Measure plasma glucose at fasting, 1-hour, and 2-hour time points. 1, 2

Diagnostic thresholds (any ONE abnormal value confirms GDM):

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

Critical Follow-Up

If GDM is diagnosed:

• Screen for persistent diabetes 4-12 weeks postpartum using a 75g OGTT with non-pregnancy diagnostic criteria. 1, 2
• Provide lifelong screening for diabetes or prediabetes at least every 3 years. 1, 2
• If prediabetes is detected postpartum, initiate intensive lifestyle interventions or metformin to prevent progression to type 2 diabetes. 1, 2