Timing of Glucose Challenge Test (GCT) for Gestational Diabetes Screening
For average-risk pregnant women, the GCT should be performed at 24-28 weeks of gestation, while high-risk women require testing as early as possible at the first prenatal visit (typically 12-14 weeks) with repeat testing at 24-28 weeks if initially negative. 1, 2
Risk-Stratified Screening Approach
High-Risk Women: Early Screening Required
High-risk women need glucose testing "as soon as feasible" after pregnancy confirmation, typically at their first prenatal visit around 12-14 weeks gestation 1, 2. High-risk characteristics include:
- Marked obesity (BMI ≥30 kg/m²) - the most significant modifiable risk factor 2, 3
- Personal history of prior GDM - confers a 4.14-fold increased risk 2
- Strong family history of diabetes in first-degree relatives 1, 2
- Glycosuria detected on routine urinalysis 1
- High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 2, 4
- History of delivering a macrosomic infant (>4.05 kg or 9 lb) 2
- Polycystic ovary syndrome (PCOS) 2
Critical caveat: If early screening is negative in high-risk women, they must be retested at 24-28 weeks, as glucose tolerance physiologically deteriorates in the third trimester 1, 2. Research confirms high rates of late-onset GDM even after normal early screening, with 17.8% of high-risk women developing GDM at 24-28 weeks despite normal initial testing 5.
Average-Risk Women: Standard Screening at 24-28 Weeks
Women of average risk should undergo GCT screening at 24-28 weeks of gestation 1, 3. This timing coincides with the peak deterioration of glucose tolerance during pregnancy, particularly in the third trimester 1.
Low-Risk Women: Screening May Be Omitted
A small subset of women meeting all of the following criteria may not require screening, though this represents a minority of patients 1:
- Age <25 years
- Normal pre-pregnancy weight (BMI <25 kg/m²)
- No first-degree family history of diabetes
- No history of abnormal glucose metabolism
- No history of poor obstetric outcomes
- Not a member of high-risk ethnic groups
Screening Methodology
Two-Step Approach (Most Common in US)
Step 1: Perform 50g GCT (non-fasting) at appropriate gestational age 1, 3
- If plasma glucose ≥140 mg/dL (7.8 mmol/L) at 1 hour, proceed to Step 2 1
- Some guidelines recommend a lower threshold of ≥130 mg/dL (7.2 mmol/L) for higher sensitivity 1
Step 2: Perform 100g OGTT (fasting) with diagnosis requiring ≥2 abnormal values 1, 3:
- Fasting: ≥95 mg/dL (5.3 mmol/L)
- 1-hour: ≥180 mg/dL (10.0 mmol/L)
- 2-hour: ≥155 mg/dL (8.6 mmol/L)
- 3-hour: ≥140 mg/dL (7.8 mmol/L)
One-Step Approach (Alternative)
Perform 75g OGTT (fasting) at 24-28 weeks with diagnosis if any one value is abnormal 1, 3:
- Fasting: ≥92 mg/dL (5.1 mmol/L)
- 1-hour: ≥180 mg/dL (10.0 mmol/L)
- 2-hour: ≥153 mg/dL (8.5 mmol/L)
The one-step approach identifies 15-20% of pregnancies as GDM compared to 5-6% with the two-step approach, though women diagnosed by one-step criteria have 3.4-fold higher risk of future diabetes 4.
Common Pitfalls to Avoid
Failing to retest high-risk women at 24-28 weeks after negative early screening is the most critical error, as research shows 38.3% of women with normal early GCT will have abnormal results at repeat testing, and 18.8% will develop late-onset GDM 5.
For women with abnormal early GCT but normal OGTT, consider proceeding directly to OGTT at 24-28 weeks without repeating the GCT, as 79.3% will have abnormal repeat GCT and 41.4% will have GDM 5.
Screening too early in average-risk women (before 24 weeks) reduces sensitivity, as glucose tolerance impairment peaks around 28 weeks of gestation 6, 7.