What are the diagnostic criteria and management options for Gestational Diabetes Mellitus (GDM)?

Gestational Diabetes Mellitus Diagnosis and Management

Gestational diabetes mellitus (GDM) should be diagnosed using either the one-step 75g OGTT or two-step approach (50g screening followed by 100g OGTT), with all pregnant women being tested at 24-28 weeks of gestation if not previously diagnosed with diabetes. 1

Diagnostic Approach

Initial Screening

• Test for undiagnosed diabetes at first prenatal visit in women with risk factors using standard diagnostic criteria 1
• Risk factors include:
  • Previous GDM
  • Obesity (BMI ≥30 kg/m²)
  • Family history of diabetes in first-degree relatives
  • High-risk ethnicity (Asian, Hispanic, African American, Native American)
  • History of macrosomia or stillbirth
  • Polycystic ovary syndrome
  • Age >35 years

GDM Diagnosis at 24-28 Weeks

Two acceptable diagnostic approaches:

One-Step Strategy (75g OGTT)

• Perform after overnight fast of at least 8 hours
• Diagnosis made when ANY of these plasma glucose values are 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) 1

Two-Step Strategy

Step 1: 50g glucose load test (non-fasting) with plasma glucose measurement at 1 hour

• If glucose ≥130-140 mg/dL (7.2-7.8 mmol/L), proceed to Step 2
• Note: ACOG recommends lower threshold of 135 mg/dL for high-risk ethnic populations 1

Step 2: 100g OGTT (fasting)

• Diagnosis made when at least two of these values are met or exceeded:

	Carpenter-Coustan Criteria
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)

Note: ACOG indicates that one elevated value can be used for diagnosis 1

Clinical Considerations for Diagnostic Approach

• The one-step approach identifies more cases of GDM (15-20% vs 5-6% with two-step) 1
• The one-step approach is based on the HAPO study showing continuous relationship between maternal glycemia and adverse outcomes 1
• The two-step approach has been the traditional method in the US and is supported by ACOG 1
• Either approach is acceptable according to ADA guidelines 1

Management of GDM

1. Lifestyle Interventions:

   • Medical nutrition therapy
   • Regular physical activity (moderate intensity if not contraindicated) 2
   • Blood glucose self-monitoring

2. Glycemic Targets:

   • Fasting: <95 mg/dL
   • 1-hour postprandial: <140 mg/dL 2

3. Pharmacologic Therapy:

   • Insulin is first-line therapy when glycemic targets not met with lifestyle modifications 2
   • Insulin does not cross the placenta in significant amounts 3
   • Metformin may be considered, though it crosses the placenta and long-term safety data in offspring are limited 4

Postpartum Follow-up

• Test for persistent diabetes or prediabetes at 4-12 weeks postpartum using 75g OGTT 1
• Lifelong screening for diabetes development at least every 3 years 1
• Women with history of GDM found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent diabetes 1

Common Pitfalls and Caveats

1. Diagnostic Confusion:

   • Pre-existing diabetes diagnosed in first trimester is not GDM but "overt diabetes in pregnancy" 1
   • Using inappropriate diagnostic thresholds for early pregnancy screening

2. Screening Challenges:

   • Missing high-risk women who should be screened at first prenatal visit
   • Improper timing of the OGTT (should be performed in morning after overnight fast)

3. Management Issues:

   • Delayed initiation of insulin when indicated
   • Inadequate postpartum follow-up (only 30-50% of women with GDM receive recommended postpartum testing)

4. Special Populations:

   • Women with history of bariatric surgery should not undergo OGTT due to risk of dumping syndrome 2
   • Lower BMI thresholds (≥23 kg/m²) should be used to define increased risk in Asian Americans 1

The diagnostic criteria for GDM continue to evolve, with ongoing debate about the optimal approach. However, early detection and appropriate management significantly reduce maternal and fetal morbidity, making universal screening at 24-28 weeks essential for all pregnant women.