How far into pregnancy can a 3-hour glucose tolerance test (GTT) be performed?

Timing of 3-Hour Glucose Tolerance Test (GTT) in Pregnancy

The 3-hour glucose tolerance test (GTT) for gestational diabetes mellitus (GDM) should be performed between 24 and 28 weeks of gestation in women not previously diagnosed with diabetes. 1

Standard Timing for GDM Testing

The timing of GDM testing is critical for optimal detection and management of glucose intolerance during pregnancy. Guidelines consistently recommend:

• Primary testing window: 24-28 weeks of gestation 1
• This timing is based on the physiologic insulin resistance that develops during the second and third trimesters

Early Testing Protocol (Before 24 Weeks)

For high-risk women, earlier testing may be appropriate:

• Test as soon as feasible at the first prenatal visit for women with:

  • Marked obesity
  • Personal history of GDM
  • Glycosuria
  • Strong family history of diabetes
  • Member of high-risk ethnic group 1

• If early testing is negative, these high-risk women should still be retested at 24-28 weeks 1

Testing After 28 Weeks

While guidelines focus on the 24-28 week window, testing after 28 weeks may still be clinically valuable:

• Testing can still be performed after 28 weeks if a woman:

  • Presents late for prenatal care
  • Develops risk factors or symptoms later in pregnancy
  • Was missed during routine screening

• However, testing much beyond 32 weeks provides less time for intervention to improve outcomes

Testing Protocol Options

The 3-hour GTT can be performed as part of either:

1. Two-step approach (most common in US):

   • Initial 50g glucose challenge test (GCT, non-fasting)
   • If GCT ≥140 mg/dL (or ≥135 mg/dL in high-risk populations), proceed to 100g 3-hour OGTT
   • Diagnosis requires at least two abnormal values 1

2. One-step approach:

   • Direct 75g 2-hour OGTT (no preliminary screening)
   • Diagnosis requires only one abnormal value 1

Important Considerations

• The 3-hour GTT should be performed in the morning after an overnight fast of 8-14 hours 1
• Patient should maintain an unrestricted diet (≥150g carbohydrate daily) for at least 3 days before testing 1
• Patient should remain seated and not smoke throughout the test 1

Pitfalls to Avoid

• Waiting too long after 28 weeks may limit time for intervention and impact maternal/fetal outcomes
• Performing the test before 24 weeks in average-risk women may miss cases that develop later due to increasing insulin resistance
• Failing to retest high-risk women at 24-28 weeks if early testing was negative 1
• Not ensuring proper fasting or dietary preparation before the test, which can affect results

In summary, while the 3-hour GTT is primarily recommended between 24-28 weeks of gestation, it can be performed earlier in high-risk women (with repeat testing at 24-28 weeks if negative) and can still provide valuable information when performed after 28 weeks in women who missed earlier screening opportunities.