What are the guidelines for screening and managing Gestational Diabetes Mellitus (GDM) using the Oral Glucose Tolerance Test (OGTT)?

Gestational Diabetes Mellitus Screening with OGTT

All pregnant women should be screened for gestational diabetes mellitus (GDM) at 24-28 weeks of gestation using either the one-step 75g OGTT or two-step approach with 50g GCT followed by 100g OGTT if positive, with high-risk women requiring additional early screening at their first prenatal visit. 1

Risk Assessment and Timing of Screening

Initial Risk Assessment

• High-risk women requiring early screening (first prenatal visit):

  • Marked obesity
  • Personal history of GDM
  • Strong family history of diabetes
  • Previous delivery of macrosomic infant
  • Glycosuria
  • Polycystic ovary syndrome (PCOS)
  • High-risk ethnic groups 1, 2

• Low-risk women (no screening required) must meet ALL criteria:

  • Age <25 years
  • Normal pre-pregnancy weight
  • Member of ethnic group with low diabetes prevalence
  • No known diabetes in first-degree relatives
  • No history of abnormal glucose tolerance
  • No history of poor obstetrical outcomes 2

• Average-risk women (all women not meeting high or low-risk criteria) should be screened at 24-28 weeks 1

Timing of Screening

• High-risk women: First prenatal visit AND again at 24-28 weeks if initial screening is negative 1
• Average-risk women: 24-28 weeks gestation 1

Screening and Diagnostic Methods

One-Step Approach (75g OGTT)

• Perform after overnight fast of at least 8 hours
• Measure plasma glucose when fasting and at 1 and 2 hours after glucose load
• GDM diagnosed when ANY ONE of these values is met or exceeded:
  • 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) 2

Two-Step Approach

1. Step 1: 50g Glucose Challenge Test (GCT) - non-fasting

   • Measure plasma glucose 1 hour after 50g glucose load
   • Proceed to diagnostic OGTT if glucose ≥130-140 mg/dL (7.2-7.8 mmol/L)
   • Using 140 mg/dL threshold identifies 80% of women with GDM
   • Using 130 mg/dL threshold increases sensitivity to 90% 2, 3

2. Step 2: 100g OGTT - performed fasting

   • GDM diagnosed when at least TWO values are met or exceeded:
     • 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)
     • 3-hour: ≥140 mg/dL (7.8 mmol/L) 2

Management of GDM

Monitoring

• Women with GDM should perform fasting and postprandial blood glucose monitoring 2
• Target glucose values:
  • Fasting plasma glucose: <95 mg/dL (<5.3 mmol/L)
  • 1-hour postprandial: <140 mg/dL (<7.8 mmol/L)
  • 2-hour postprandial: <120 mg/dL (<6.7 mmol/L) 2

Treatment

• First-line: Lifestyle modifications including dietary changes and moderate physical activity 1
• Second-line: Insulin therapy when lifestyle modifications fail to maintain target glucose levels 1, 4
• Insulin recommended when medical nutrition therapy fails to maintain:
  • Fasting plasma glucose <105 mg/dL (5.8 mmol/L)
  • 1-hour postprandial plasma glucose <155 mg/dL (8.6 mmol/L)
  • 2-hour postprandial plasma glucose <130 mg/dL (7.2 mmol/L) 2

Postpartum Follow-up

• All women with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using the 75g OGTT 2, 1, 4
• Lifelong screening for diabetes development should be performed at least every 3 years 2, 1

Potential Pitfalls and Considerations

• Pre-analytical sample handling: Glucose levels can decrease due to glycolysis if samples are not properly handled. Samples should be analyzed promptly or placed in appropriate preservative tubes 5
• Improper test preparation: Inadequate fasting or improper carbohydrate intake can affect OGTT results 1
• Missing high-risk women: Failing to identify and screen high-risk women early can delay diagnosis and treatment 1
• Overlooking postpartum screening: Can lead to delayed diagnosis of type 2 diabetes in women with GDM history 1
• Assuming normal early screening eliminates need for 24-28 week testing: High-risk women with negative early screening still need testing at 24-28 weeks 1

Clinical Impact of GDM

Women with GDM are at increased risk for developing type 2 diabetes after pregnancy, while their offspring have increased risk of obesity, glucose intolerance, and diabetes in adolescence and young adulthood 2. Early detection and management of GDM reduces the risk of adverse maternal and fetal outcomes including preeclampsia, macrosomia, and shoulder dystocia 1.