What is the glucose dose (in grams) for a 3‑hour oral glucose tolerance test during pregnancy?

3-Hour GTT in Pregnancy: 100 Grams of Glucose

The 3-hour oral glucose tolerance test (OGTT) during pregnancy uses 100 grams of glucose. 1121

Test Protocol

The 3-hour 100-gram OGTT is part of the two-step approach for diagnosing gestational diabetes mellitus (GDM):

Step 1: Initial Screening

• 50-gram glucose load test (GLT), non-fasting
• Plasma glucose measured at 1 hour
• Performed at 24-28 weeks gestation
• If ≥140 mg/dL (or ≥130-135 mg/dL depending on local practice), proceed to Step 2 11

Step 2: Diagnostic Test (The 3-Hour Test)

• 100-gram glucose load
• Patient must be fasting (at least 8 hours)
• Plasma glucose measured at: fasting, 1 hour, 2 hours, and 3 hours
• GDM diagnosed when at least 2 of 4 values meet or exceed thresholds 112

Diagnostic Thresholds for 100g 3-Hour OGTT

Two sets of criteria are recognized:

Carpenter/Coustan criteria:

• Fasting: ≥95 mg/dL
• 1 hour: ≥180 mg/dL
• 2 hours: ≥155 mg/dL
• 3 hours: ≥140 mg/dL 112

NDDG criteria (alternative):

• Fasting: ≥105 mg/dL
• 1 hour: ≥190 mg/dL
• 2 hours: ≥165 mg/dL
• 3 hours: ≥145 mg/dL 11

Important Clinical Context

The two-step approach with the 100-gram 3-hour test is endorsed by ACOG and remains widely used in the United States 13. The alternative one-step approach uses a 75-gram 2-hour OGTT (not 3-hour), which is a completely different protocol 114.

Note: Recent ACOG guidance suggests that even one elevated value (rather than two) on the 100g test may be sufficient for diagnosis, which would increase GDM detection rates 3. This reflects emerging evidence that even a single abnormal value is associated with adverse outcomes comparable to frank GDM 5.