Gestational Diabetes Screening: 50-g Glucose Challenge Test
Timing of the 50-g Glucose Challenge Test
The 50-g oral glucose challenge test (GCT) should be administered at 24–28 weeks gestation as the first step of the two-step screening approach, with a plasma glucose threshold of ≥140 mg/dL (or ≥130 mg/dL for higher sensitivity) triggering diagnostic testing. 1
Standard Screening Window (24–28 Weeks)
All pregnant women without known pre-existing diabetes should undergo universal screening at 24–28 weeks gestation, the period of maximal pregnancy-related insulin resistance when gestational diabetes most commonly manifests. 1
The 50-g GCT is administered without fasting at any time of day, making it more convenient and better tolerated than the diagnostic oral glucose tolerance test (OGTT). 1, 2
Plasma glucose is measured once at 1 hour after the 50-g oral glucose load. 1, 3
Early Screening for High-Risk Women (First Prenatal Visit, ~12–14 Weeks)
Women with marked obesity (BMI ≥30 kg/m²), personal history of gestational diabetes, glycosuria, or strong family history of diabetes should undergo glucose testing at the first prenatal visit to detect undiagnosed type 2 diabetes. 1, 2, 4
If early screening is negative in high-risk women, repeat screening at 24–28 weeks is mandatory because insulin resistance increases exponentially in the second and third trimesters. 1, 2
Abnormal GCT Thresholds and Diagnostic Follow-Up
Two-Step Approach (ACOG-Supported)
Step 1: 50-g Glucose Challenge Test
A 1-hour plasma glucose ≥140 mg/dL is the standard threshold for an abnormal screen, triggering diagnostic testing. 1
Some centers use a lower threshold of ≥130 mg/dL to increase sensitivity (99% vs. 85%), though this reduces specificity (77% vs. 86%). 1
The 140 mg/dL threshold offers the best balance of sensitivity (85%) and specificity (86%) when using Carpenter-Coustan diagnostic criteria. 1
Step 2: 100-g Oral Glucose Tolerance Test (Diagnostic)
Women with an abnormal GCT proceed to a 100-g OGTT performed after an 8–14 hour overnight fast with plasma glucose measured at fasting, 1 hour, 2 hours, and 3 hours. 1, 5
Gestational diabetes is diagnosed when at least two of the following Carpenter-Coustan thresholds are met or exceeded:
ACOG notes that in clinical practice, a single elevated value may be used for diagnosis, though traditional criteria require two abnormal values. 5
When the GCT Alone Is Diagnostic
When the 1-hour GCT value is ≥200 mg/dL (11.1 mmol/L), the positive predictive value for gestational diabetes exceeds 85%, and some experts consider this diagnostic without confirmatory OGTT. 6
A GCT value ≥216 mg/dL (12.0 mmol/L) has a positive predictive value >95% for gestational diabetes, particularly when performed in the morning. 6
Alternative One-Step Approach (IADPSG/ADA)
The 75-g OGTT performed at 24–28 weeks (fasting, with measurements at fasting, 1 hour, and 2 hours) is an alternative to the two-step approach. 1, 5, 4
Diagnosis requires any single abnormal value:
The one-step approach identifies approximately 15–20% of pregnancies with gestational diabetes, compared to 5–6% with the two-step method, capturing milder hyperglycemia that still benefits from treatment. 5
Test Preparation for Diagnostic OGTT
The diagnostic 100-g OGTT must be performed in the morning after an 8–14 hour overnight fast. 1, 5
Patients should consume ≥150 g of carbohydrate daily for at least 3 days before the test to ensure adequate glycogen stores. 1, 5
Physical activity should be unrestricted in the days preceding the test, the patient should remain seated throughout the OGTT, and smoking is prohibited during testing. 1, 5
Management After Diagnosis
Immediate Treatment
All women diagnosed with gestational diabetes should receive nutritional counseling, blood glucose self-monitoring instruction, and encouragement to increase physical activity to moderate intensity levels if not contraindicated. 7
Insulin therapy is first-line pharmacologic treatment when blood glucose levels cannot be maintained in the therapeutic range (fasting <95 mg/dL and 1-hour postprandial <140 mg/dL). 1, 7
Postpartum Follow-Up
All women with gestational diabetes must undergo a 75-g OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes. 1, 2, 4
Lifelong diabetes screening every 3 years is mandatory, as women with prior gestational diabetes have a 3.4-fold increased risk of developing type 2 diabetes. 2, 4
Women identified with postpartum prediabetes should receive intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes. 2, 4
Critical Pitfalls 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. 1
Do not skip repeat screening at 24–28 weeks in high-risk women who had a negative early screen, as this is the most common error leading to missed diagnoses. 2
Do not use hemoglobin A1c for gestational diabetes screening, as it has poor sensitivity and specificity; A1c ≥6.5% may identify pre-existing diabetes but is not validated for GDM detection. 2, 7
Do not rely on urine glucose testing for GDM detection or management, as the renal glucose threshold decreases during pregnancy. 1, 5
Do not apply gestational diabetes diagnostic thresholds (92/180/153 mg/dL) to early-pregnancy screening; use non-pregnancy criteria (fasting ≥126 mg/dL) to detect overt diabetes. 2, 5