At what gestational age should the oral glucose tolerance test (OGTT) be performed to screen for gestational diabetes mellitus, and does this differ for women with risk factors?

Timing of Glucose Tolerance Testing in Pregnancy

All pregnant women without pre-existing diabetes should undergo gestational diabetes mellitus (GDM) screening at 24–28 weeks of gestation using either a one-step 75-g OGTT or a two-step approach (50-g glucose challenge followed by 100-g OGTT if abnormal). 1, 2

Standard Universal Screening (24–28 Weeks)

This timing window represents the period of maximal pregnancy-related insulin resistance when GDM most commonly manifests. 1, 2

Two acceptable screening strategies exist:

One-Step Approach (IADPSG/ADA Criteria)

• Perform a 75-g OGTT after an 8–14 hour overnight fast with plasma glucose measured at fasting, 1 hour, and 2 hours. 1, 2
• Diagnosis requires only ONE abnormal value: fasting ≥92 mg/dL (5.1 mmol/L), 1-hour ≥180 mg/dL (10.0 mmol/L), or 2-hour ≥153 mg/dL (8.5 mmol/L). 1, 2
• This approach identifies approximately 15–20% of pregnancies with GDM. 2
• These thresholds derive from the HAPO study, which demonstrated continuous increases in adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) as maternal glucose rises, with no clear safety threshold. 1, 2

Two-Step Approach (ACOG-Supported)

• Step 1: Administer a 50-g non-fasting glucose challenge test at 24–28 weeks; if 1-hour plasma glucose is ≥130–140 mg/dL, proceed to Step 2. 1, 2
• Step 2: Perform a 100-g OGTT after an overnight fast with measurements at fasting, 1,2, and 3 hours. 1, 2
• Diagnosis requires at least TWO abnormal values (Carpenter-Coustan criteria): 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), or 3-hour ≥140 mg/dL (7.8 mmol/L). 1, 2
• ACOG notes that a single elevated value may be used for diagnosis in clinical practice, though traditional criteria require two abnormal values. 1, 2
• This approach identifies approximately 5–6% of pregnancies with GDM. 2

The one-step method identifies roughly twice as many women with GDM compared to the two-step method, capturing milder hyperglycemia that still benefits from treatment. 1, 2

Early Screening for High-Risk Women (First Prenatal Visit, ~12–14 Weeks)

Women meeting ANY of the following high-risk criteria should undergo glucose testing at the first prenatal visit to detect pre-existing undiagnosed diabetes:

• BMI ≥30 kg/m² (the single most important risk factor) 2, 3
• Personal history of prior GDM (4.14-fold increased risk) 3, 4
• First-degree relative with diabetes 1, 2, 3
• Glycosuria on routine urinalysis 3
• High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 2, 3
• History of delivering a macrosomic infant (>4,500 g) 3
• History of polycystic ovary syndrome 3

Early Testing Thresholds (Detecting Overt Diabetes, Not GDM)

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) indicates overt pre-existing diabetes. 2, 3
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with hyperglycemia symptoms indicates overt diabetes. 2, 3
• HbA1c ≥6.5% indicates overt diabetes (though HbA1c is not recommended for routine GDM screening due to limited sensitivity). 2, 5
• Fasting glucose 92–125 mg/dL in early pregnancy meets GDM diagnostic criteria. 2

Critical: If early screening is negative in high-risk women, mandatory repeat testing at 24–28 weeks is required because insulin resistance increases exponentially in the second and third trimesters. 2, 3, 4 This is the most common clinical error—failing to retest high-risk women after a negative early screen leads to missed GDM diagnoses. 2

Test Preparation Requirements

• The OGTT must be performed in the morning after an 8–14 hour overnight fast. 1, 2
• Patients should consume ≥150 g of carbohydrate daily for at least 3 days before testing to ensure adequate glycogen stores. 2
• Physical activity should remain unrestricted in the days preceding the test. 2
• The patient must remain seated throughout the entire OGTT and must not smoke during testing. 2

Women Who May Skip Screening

Only women meeting ALL of the following low-risk criteria may forgo GDM screening entirely:

• Age <25 years 1, 2, 3
• Pre-pregnancy BMI ≤25 kg/m² 1, 2, 3
• No first-degree relative with diabetes 1, 2, 3
• No history of abnormal glucose tolerance 1, 2, 3
• No history of poor obstetrical outcomes 1, 2, 3
• Member of an ethnic group with low diabetes prevalence 1, 2, 3

In practice, universal screening at 24–28 weeks is now standard because the vast majority of pregnant women possess at least one risk factor. 2, 3

Critical Pitfalls to Avoid

• Do not apply the GDM diagnostic thresholds (92/180/153 mg/dL) to early-pregnancy screening—early testing should use standard non-pregnancy diabetes criteria (fasting ≥126 mg/dL or random ≥200 mg/dL). 2
• Do not postpone screening beyond 28 weeks—the optimal 24–28 week window coincides with peak insulin resistance and allows timely intervention to reduce adverse outcomes. 1, 2
• Do not rely on urine glucose testing for GDM detection or management—it is not useful. 2
• Do not perform OGTT after bariatric surgery due to risk of postprandial hypoglycemia; use alternative monitoring strategies. 5
• Do not use HbA1c for routine GDM screening—it has insufficient sensitivity for detecting gestational hyperglycemia. 6, 5

Postpartum and Long-Term Follow-Up

• All women diagnosed with GDM must undergo a 75-g OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to identify persistent diabetes or prediabetes. 1, 2, 4
• Lifelong diabetes screening every 2–3 years is mandatory for women with prior GDM, as they have a 3.4-fold increased risk of developing type 2 diabetes. 1, 2, 4
• Women with postpartum prediabetes should receive intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes. 1, 2