Gestational Diabetes Screening at 23 Weeks Gestation
This patient should undergo a 75-gram oral glucose tolerance test (OGTT) at 24–28 weeks gestation, not at her current 23-week visit. 1, 2
Why Not Screen at 23 Weeks?
The 24–28 week window is mandatory because this period corresponds to peak pregnancy-related insulin resistance, when gestational diabetes most frequently manifests and when diagnostic criteria have been validated. 1, 2
The IADPSG and Carpenter-Coustan diagnostic thresholds should not be applied before 24 weeks gestation, as they were derived exclusively from data collected in the later half of pregnancy. 1, 3
The HAPO study, which established current diagnostic cut-offs, evaluated glucose-outcome relationships specifically at 24–28 weeks, not earlier. 1, 2
Does Family History of Type 2 Diabetes Change the Timing?
No—a single first-degree relative with diabetes does not justify early screening at 23 weeks. 2
Early screening at the first prenatal visit (12–14 weeks) is reserved for very high-risk women with marked obesity (BMI ≥30 kg/m²), prior gestational diabetes, glycosuria, or multiple first-degree relatives with early-onset diabetes. 2, 3
A family history of type 2 diabetes in one parent elevates risk but places this patient in the average-risk category, which requires standard screening at 24–28 weeks only. 2, 3
Why the Other Options Are Incorrect
A. Hemoglobin A1C at This Visit
HbA1c is not recommended for gestational diabetes screening due to poor sensitivity and specificity for detecting GDM. 1, 2
HbA1c may only be used to identify pre-existing type 2 diabetes early in pregnancy (threshold ≥6.5%), not for routine GDM screening. 1, 2
B. Random Blood Sugar at This Visit
Random plasma glucose measurements are not validated or recommended for GDM screening. 2
Random glucose ≥200 mg/dL with symptoms indicates overt diabetes, not screen-positive GDM, and would require immediate diagnostic confirmation with fasting glucose or HbA1c. 1, 2
D. 3-Hour Glucose Tolerance Test at 28–32 Weeks
The 3-hour 100-gram OGTT is a diagnostic test used only in the two-step approach after a positive 50-gram glucose challenge test; it is not a primary screening tool. 1, 2
The timing of 28–32 weeks is suboptimal; the recommended window is 24–28 weeks to allow timely intervention before the third trimester. 1, 2
The Correct Answer: C. 75-Gram OGTT at 24–28 Weeks (Corrected Timing)
The patient should return at 24–28 weeks for a 75-gram OGTT using the one-step approach endorsed by the American Diabetes Association and International Association of the Diabetes and Pregnancy Study Groups. 1, 2
Test Procedure
Perform after an 8–14 hour overnight fast with plasma glucose measured at fasting, 1 hour, and 2 hours. 1, 3
Diagnosis requires any single abnormal value:
Patients should consume ≥150 grams of carbohydrate daily for at least 3 days before the test to ensure adequate glycogen stores. 3
Alternative Two-Step Approach
The American College of Obstetricians and Gynecologists supports a two-step strategy:
ACOG notes that in clinical practice a single elevated value may be used for diagnosis, although traditional Carpenter-Coustan criteria require two abnormal values. 1, 3
Critical Pitfall to Avoid
Do not postpone screening beyond 28 weeks. The 24–28 week window enables timely intervention to reduce preeclampsia, macrosomia, shoulder dystocia, and neonatal hypoglycemia—outcomes that worsen with delayed diagnosis. 1, 2
Postpartum Follow-Up
All women diagnosed with GDM must undergo a 75-gram OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes. 1, 4
Lifelong diabetes screening every 3 years is mandatory because women with prior GDM have a 3.4-fold increased risk of developing type 2 diabetes. 1, 4
Women with postpartum prediabetes should receive intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes. 1, 4