What test should be ordered for a 16-week primigravida with a BMI of 35 to screen for gestational diabetes mellitus?

Early Screening for Gestational Diabetes in High-Risk Pregnancy

Order a fasting blood glucose (or fasting plasma glucose as part of a 75g OGTT) at this 16-week visit, as this patient's BMI of 35 kg/m² places her in the high-risk category requiring immediate screening for undiagnosed pre-existing type 2 diabetes. 1, 2

Rationale for Early Screening

• Women with BMI ≥30 kg/m² should be screened at their first prenatal visit (typically 12-14 weeks) to detect pre-existing undiagnosed type 2 diabetes, not just gestational diabetes. 1, 3 At 16 weeks, this patient is still within the appropriate window for this initial screening.

• The American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians all recommend early screening for pregnant women with obesity (BMI ≥30 kg/m²) due to significantly higher risk of both pre-existing diabetes and GDM. 4, 1

• Her concern about macrosomia is clinically relevant, as maternal hyperglycemia directly correlates with fetal macrosomia, shoulder dystocia, and other adverse outcomes. 4

Specific Test to Order at 16 Weeks

Option A (Fasting Blood Glucose) is the correct initial choice at this gestational age for this patient. 4, 3

• Fasting plasma glucose ≥126 mg/dL indicates overt diabetes requiring immediate treatment. 4, 3

• Fasting glucose ≥92 mg/dL but <126 mg/dL suggests early GDM or impaired glucose metabolism requiring close monitoring. 4, 3

• Option B (75g glucose challenge test) is not the standard terminology—the correct tests are either a 50g glucose load test (GLT) or a 75g oral glucose tolerance test (OGTT), not a "75g glucose challenge test." 4

Complete Screening Algorithm for This Patient

At Current Visit (16 weeks):

• Perform fasting plasma glucose testing. 1, 3
• If fasting glucose ≥126 mg/dL: diagnose overt diabetes and initiate treatment immediately. 4, 3
• If fasting glucose 92-125 mg/dL: consider early GDM, initiate dietary counseling and glucose monitoring. 3
• If fasting glucose <92 mg/dL: proceed with routine screening at 24-28 weeks. 1

At 24-28 Weeks (Mandatory Repeat Screening):

Even if initial screening is negative, this patient MUST be rescreened at 24-28 weeks due to her high-risk status. 1, 2

You have two evidence-based options:

Two-Step Approach (most commonly used in US): 4, 2

• Step 1: 50g glucose load test (non-fasting), measure glucose at 1 hour
• If ≥130-140 mg/dL, proceed to Step 2
• Step 2: 100g OGTT (fasting) with measurements at 0,1,2, and 3 hours
• Diagnosis requires ≥2 abnormal values: fasting ≥95 mg/dL, 1-hr ≥180 mg/dL, 2-hr ≥155 mg/dL, 3-hr ≥140 mg/dL 4, 2

One-Step Approach (international consensus): 4, 2

• 75g OGTT (fasting) with measurements at 0,1, and 2 hours
• Diagnosis requires only ONE abnormal value: fasting ≥92 mg/dL, 1-hr ≥180 mg/dL, or 2-hr ≥153 mg/dL 4, 2

Critical Pitfalls to Avoid

• Failing to rescreen at 24-28 weeks if early screening is negative—GDM typically develops in the second/third trimester due to increasing insulin resistance, so a negative early test does not exclude later development. 1

• Using HbA1c for GDM screening at 24-28 weeks—HbA1c does not perform as well as glucose tolerance testing for GDM diagnosis. 4

• Assuming early screening alone is sufficient—early screening identifies pre-existing diabetes, while 24-28 week screening identifies true gestational diabetes. 1

Postpartum Follow-up

• Screen for persistent diabetes at 4-12 weeks postpartum with 75g OGTT using non-pregnancy diagnostic criteria. 4, 2

• Lifelong screening for diabetes or prediabetes at least every 3 years thereafter. 4, 2

• If prediabetes is identified, initiate intensive lifestyle interventions or metformin for diabetes prevention. 4