Should I order a 75‑gram oral glucose tolerance test (OGTT) for a pregnant patient?

Should You Order a 75-gram OGTT for a Pregnant Patient?

Yes, you should order a 75-gram OGTT for all pregnant patients without known diabetes at 24-28 weeks of gestation to screen for gestational diabetes mellitus (GDM), as this is the standard of care recommended by the American Diabetes Association. 1

Screening Approach: Two Options Available

The ADA recognizes two acceptable strategies for GDM screening, though both involve testing at 24-28 weeks gestation 1:

One-Step Strategy (75-gram OGTT)

• Perform a single 75-gram OGTT with plasma glucose measurements at fasting, 1-hour, and 2-hour timepoints 1
• The test must be done in the morning after an overnight fast of at least 8 hours 1
• GDM is diagnosed if ANY ONE of the following values is met or exceeded: 1
  • 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)

Two-Step Strategy (Alternative)

• Step 1: 50-gram glucose load test (non-fasting) at 24-28 weeks 1
• If 1-hour glucose is ≥130,135, or 140 mg/dL, proceed to Step 2 1
• Step 2: 100-gram OGTT (fasting) with measurements at 0,1,2, and 3 hours 1
• GDM diagnosed if at least two values meet or exceed thresholds (Carpenter-Coustan criteria) 1

Key Clinical Considerations

Why This Matters for Outcomes

The landmark HAPO study of over 23,000 pregnant women demonstrated that maternal and fetal adverse outcomes increase continuously with maternal glycemia at 24-28 weeks, even within previously "normal" ranges, with no clear threshold for risk 1. This evidence base directly informed current diagnostic criteria to optimize pregnancy outcomes rather than simply predict future maternal diabetes 1.

Early Pregnancy Screening

Consider early screening (before 24 weeks) in high-risk populations with risk factors for undiagnosed type 2 diabetes 1. However, use standard non-pregnancy diagnostic criteria (fasting glucose ≥126 mg/dL or A1C ≥6.5%) for early screening, not the 75-gram OGTT criteria, as these were not validated for early pregnancy 1.

If early screening is negative, you must still rescreen at 24-28 weeks using the standard GDM criteria 1.

Common Pitfall to Avoid

The one-step approach identifies approximately 15-20% of pregnant women with GDM (versus 5-6% with older criteria), primarily because only one abnormal value is required rather than two 1. This is not over-diagnosis—these women have demonstrable increased risk for adverse outcomes including macrosomia, preeclampsia, and perinatal complications 1. Importantly, 80-90% of women diagnosed with mild GDM can be managed with lifestyle therapy alone 1.

Practical Implementation

• The 75-gram OGTT requires fasting but is completed in one visit over 2 hours 1
• The two-step approach doesn't require initial fasting but approximately 15-20% of women need the second 3-hour test 2
• Both approaches are acceptable; the ADA recommends the IADPSG one-step criteria as they are the only criteria based on pregnancy outcomes rather than prediction of subsequent maternal diabetes 1

Postpartum Follow-up

Women diagnosed with GDM should receive a 75-gram OGTT at 4-12 weeks postpartum using non-pregnancy diagnostic criteria to assess for persistent diabetes or prediabetes 1. They require lifelong screening every 1-3 years as they have a 50-60% lifetime risk of developing type 2 diabetes 1.