When is gestational diabetes typically checked during pregnancy?

Gestational Diabetes Screening Timeline

Gestational diabetes screening should be performed at 24-28 weeks of gestation for women of average risk, while high-risk women should be screened at their first prenatal visit and again at 24-28 weeks if initial results are negative. 1

Risk Assessment and Timing

First Prenatal Visit

• All pregnant women: Risk assessment should be performed at the first prenatal visit 1
• High-risk women: Should undergo glucose testing as soon as feasible 1
  • High-risk characteristics include:
    • Marked obesity
    • Personal history of GDM
    • Glycosuria
    • Strong family history of diabetes
    • Polycystic ovary syndrome (PCOS)
    • Previous delivery of a large-for-gestational-age infant (>4.05 kg or 9 lb)
    • Member of high-risk ethnic group (Hispanic, Native American, South or East Asian, African American, Pacific Islander) 1

24-28 Weeks Gestation

• Average-risk women: Should be screened at 24-28 weeks 1
• High-risk women with negative initial screening: Should be retested at 24-28 weeks 1

Low-Risk Women (May Skip Screening)

Women meeting ALL of the following criteria may not require screening 1:

• Age <25 years
• Normal pre-pregnancy weight
• No family history of diabetes in first-degree relatives
• No history of abnormal glucose metabolism
• No history of poor obstetric outcomes
• Not a member of an ethnic/racial group with high diabetes prevalence

Screening Methods

Two-Step Approach (Most Common in US)

1. Initial screening: 50g glucose challenge test (GCT) in non-fasting state
   • Positive threshold: ≥140 mg/dL (identifies 80% of GDM cases)
   • Alternative threshold: ≥130 mg/dL (increases sensitivity to 90%) 1
2. Diagnostic test: For those exceeding threshold, perform 100g oral glucose tolerance test (OGTT)
   • Diagnosis requires two or more abnormal values:
     • Fasting: ≥95 mg/dL
     • 1-hour: ≥180 mg/dL
     • 2-hour: ≥155 mg/dL
     • 3-hour: ≥140 mg/dL 1

One-Step Approach

• Perform diagnostic 75g OGTT without prior screening
• May be more cost-effective in high-risk populations
• Diagnosis requires one or more abnormal values:
  • Fasting: ≥92 mg/dL
  • 1-hour: ≥180 mg/dL
  • 2-hour: ≥153-155 mg/dL 1, 2

Important Clinical Considerations

Early Screening Outcomes

• Women diagnosed with GDM before 24 weeks are more likely to require insulin therapy 3
• However, early vs. standard screening and treatment in high-risk women shows similar short-term maternal-fetal outcomes 3

Post-Partum Follow-up

• All women with GDM should be screened for persistent diabetes at 6-12 weeks postpartum 1
• Subsequent screening for diabetes development should occur every 2-3 years 1, 2

Common Pitfalls to Avoid

1. Missing high-risk women who need early screening: Failing to identify women who should be screened at their first prenatal visit can delay diagnosis and treatment
2. Improper test preparation: OGTT should be performed in the morning after 8-14 hours of fasting and at least 3 days of unrestricted diet (≥150g carbohydrate daily) 1
3. Overlooking postpartum screening: Women with GDM have a significantly increased risk of developing type 2 diabetes later in life 1
4. Assuming normal early screening eliminates need for 24-28 week testing: High-risk women with normal early results still require standard screening at 24-28 weeks 1

The evidence clearly demonstrates that timely screening and treatment of GDM significantly reduces maternal and fetal complications, including preeclampsia, shoulder dystocia, and macrosomia 1, 4.