A 23‑week pregnant G2P1 with normal evaluation, O‑positive blood type, and a mother with type 2 diabetes is concerned about developing diabetes; what is the most appropriate next step in management?

Screening for Gestational Diabetes in a 23-Week Pregnant Woman with Family History of Type 2 Diabetes

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

Risk Assessment and Timing

This patient has a significant risk factor—a first-degree relative with type 2 diabetes—which places her at higher than low-risk status for gestational diabetes mellitus (GDM). 1 However, she does not meet criteria for immediate early testing, which is reserved for women with marked obesity, personal history of GDM, glycosuria, or a strong family history (typically interpreted as multiple first-degree relatives or early-onset diabetes in parents). 1

The distinction between screening for GDM versus pre-existing type 2 diabetes is critical here:

• Women at very high risk (marked obesity, prior GDM, glycosuria, or strong family history) should undergo glucose testing at the first prenatal visit to detect unrecognized pre-existing diabetes. 1, 2
• If early testing is negative or not performed, these women must be retested at 24-28 weeks for GDM. 1, 2
• Women of average risk (which includes those with a single first-degree relative with diabetes but no other major risk factors) should have testing undertaken at 24-28 weeks. 1

Why Not the Other Options

Option A (HbA1c now): HbA1c is not recommended for GDM screening or diagnosis during pregnancy. 3 While HbA1c can detect pre-existing diabetes in early pregnancy, this patient is already at 23 weeks and has had unremarkable testing thus far. 4 The OGTT is more sensitive for detecting glucose intolerance in pregnancy. 1

Option B (Random blood glucose now): Random glucose measurements are not validated for GDM screening or diagnosis. 3 A fasting plasma glucose ≥126 mg/dL or random glucose ≥200 mg/dL would indicate overt diabetes, but these are not screening tests. 1

Option D (3-hour OGTT 24-28 weeks): The 3-hour 100-gram OGTT is the diagnostic test used in the two-step approach, not the initial screening test. 1 It is only performed after a positive 50-gram glucose challenge test (GCT) in the two-step protocol. 1, 2

Recommended Approach

At 24-28 weeks, this patient should undergo either:

1. One-step approach (preferred by ADA/IADPSG): A 75-gram OGTT with measurements at fasting, 1-hour, and 2-hour. 1, 2 GDM is diagnosed if any single value meets or exceeds: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL. 1, 2

2. Two-step approach (supported by ACOG): A 50-gram non-fasting glucose challenge test followed by a 100-gram 3-hour OGTT if the 1-hour value is ≥140 mg/dL (or ≥130 mg/dL for increased sensitivity). 1, 2 The 100-gram OGTT diagnoses GDM when at least two of four values are elevated. 1

Test Preparation

The OGTT must be performed after an overnight fast of 8-14 hours, with at least 150 grams of carbohydrate intake daily for 3 days prior to testing. 1, 2 The patient should remain seated throughout the test and not smoke. 1, 2

Common Pitfalls to Avoid

• Do not delay testing beyond 28 weeks. While late OGTT can still diagnose GDM, optimal timing is 24-28 weeks when insulin resistance peaks and treatment can prevent adverse outcomes. 1, 2
• Do not rely on urine glucose testing, which is not useful for GDM management. 2
• Do not assume a family history alone warrants immediate testing at 23 weeks unless other high-risk features are present. 1, 2