What screening test should be ordered for a 16-week primigravida (first pregnancy) woman with a body mass index (BMI) of 35 and concerns about having a macrosomic (excessively large) baby?

Screening Test for Gestational Diabetes at 16 Weeks in High-Risk Patient

Order a 50-gram glucose challenge test (GCT) now at 16 weeks, followed by a diagnostic 100-gram oral glucose tolerance test if abnormal, and mandatory repeat screening at 24-28 weeks regardless of initial results. 1

Rationale for Early Screening

This patient requires immediate screening due to her significantly elevated risk profile:

• BMI of 35 places her at substantially elevated risk for both gestational diabetes and fetal macrosomia, with untreated gestational diabetes carrying up to 20% risk of macrosomia 1, 2
• Women with BMI ≥30 kg/m² should be screened at their first prenatal visit (12-14 weeks) and again at 24-28 weeks if initially negative 1
• At 16 weeks, she is already past the ideal early screening window but should be tested immediately 1

Recommended Testing Approach

Initial screening test:

• Start with the 50-gram glucose challenge test (GCT) in a non-fasting state 1
• This is the most commonly used screening test in the United States 3
• If the result is ≥130-140 mg/dL, proceed to diagnostic testing 1

Diagnostic confirmation if screening positive:

• Perform a 100-gram oral glucose tolerance test (OGTT) for definitive diagnosis 1
• Alternatively, a 75-gram OGTT may be used directly without prior screening in high-risk populations, which can be more cost-effective 1

Why Not Fasting Blood Glucose Alone?

Fasting glucose alone is insufficient for gestational diabetes diagnosis - a full OGTT is required for definitive diagnosis 1. Relying solely on fasting glucose will miss many cases of gestational diabetes, particularly in obese women where postprandial glucose abnormalities may predominate.

Critical Follow-Up Requirements

Mandatory repeat screening at 24-28 weeks is essential even if initial testing is negative 1:

• Insulin resistance increases exponentially in the second and third trimesters 1
• Failing to rescreen high-risk women who initially test negative leads to delayed diagnosis and increased maternal-fetal complications 1
• This is the most common pitfall to avoid in early screening protocols 1

Clinical Context

Early screening allows prompt intervention if glucose intolerance is detected, reducing complications 1. In this patient:

• Obesity independently increases macrosomia risk beyond diabetes effects 1
• Undiagnosed gestational diabetes in obese women carries macrosomia rates up to 20% 1, 2
• Early diagnosis may actually represent pregestational diabetes that was unrecognized before pregnancy 3

Standard Screening After 24 Weeks

All pregnant women should be screened for gestational diabetes after 24 weeks of gestation 3. The USPSTF gives this a B recommendation with adequate evidence that treatment of screen-detected gestational diabetes reduces preeclampsia, fetal macrosomia, and shoulder dystocia 3. Treatment provides moderate net benefit for mother and infant when these outcomes are considered collectively 3.

Common Pitfalls to Avoid

• Do not delay screening - this patient is already at 16 weeks and should be tested immediately 1
• Do not skip the 24-28 week rescreen if early testing is negative 1
• Do not rely on fasting glucose alone for diagnosis 1
• Do not assume a negative early test excludes gestational diabetes - insulin resistance peaks later in pregnancy 1