What is the diagnosis and management of gestational diabetes mellitus (GDM)?

Diagnosis of Gestational Diabetes Mellitus (GDM)

The diagnosis of gestational diabetes mellitus (GDM) can be made using either a one-step or two-step approach, with screening recommended at 24-28 weeks of gestation for all pregnant women not previously diagnosed with diabetes. 1

Screening Recommendations

• Test for undiagnosed diabetes at the first prenatal visit in women with risk factors using standard diagnostic criteria 1
• Test for GDM at 24-28 weeks of gestation in pregnant women not previously known to have diabetes 1
• Test women with GDM for prediabetes or diabetes at 4-12 weeks postpartum using the 75-g oral glucose tolerance test (OGTT) and clinically appropriate nonpregnancy diagnostic criteria 1
• Women with a history of GDM should have lifelong screening for diabetes or prediabetes at least every 3 years 1

Diagnostic Approaches

One-Step Strategy (IADPSG Criteria)

• Perform a 75-g OGTT with plasma glucose measurement when fasting and at 1 and 2 hours, at 24-28 weeks of gestation 1
• The OGTT should be performed in the morning after an overnight fast of at least 8 hours 1
• The diagnosis of GDM is made when ANY of the following 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, 2

Two-Step Strategy

Step 1:

• Perform a 50-g glucose load test (GLT) (nonfasting), with plasma glucose measurement at 1 hour, at 24-28 weeks of gestation 1
• If the plasma glucose level measured 1 hour after the load is ≥130,135, or 140 mg/dL (7.2,7.5, or 7.8 mmol/L, respectively), proceed to a 100-g OGTT 1

Step 2:

• The 100-g OGTT should be performed when the patient is fasting 1
• The diagnosis of GDM is made when at least two of the following four plasma glucose levels (measured fasting and at 1,2, and 3 hours during OGTT) 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) 1

Clinical Considerations and Controversies

• The one-step method identifies approximately 2-3 times more cases of GDM than the two-step approach (prevalence of 15-20% versus 5-6%) 2, 3
• The American College of Obstetricians and Gynecologists (ACOG) currently supports the two-step approach but notes that one elevated value can be used for diagnosis 1, 2
• The one-step method identifies women at long-term risk of maternal prediabetes and diabetes and offspring abnormal glucose metabolism and adiposity 1, 3
• The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Follow-up Study demonstrated that maternal glycemia is associated with immediate and long-term adverse outcomes 1, 4
• The choice between approaches remains controversial and may depend on factors such as population prevalence, available resources, and cost-effectiveness considerations 2, 3

Common Pitfalls and Caveats

• Failure to screen at the appropriate gestational age (24-28 weeks) may miss cases of GDM 1
• Using incorrect diagnostic thresholds or requiring the wrong number of abnormal values can lead to misdiagnosis 1
• Not performing the OGTT after an overnight fast of at least 8 hours can affect results 1
• Some studies suggest that all women with a 50-g screen value >216 mg/dL have evidence of GDM and may not require a confirmatory 3-hour OGTT 5
• Failure to follow up with postpartum testing misses the opportunity to identify women at high risk for developing type 2 diabetes 1, 6

Post-GDM Follow-up

• Women with GDM should be tested for persistent diabetes at 4-12 weeks postpartum using a 75-g OGTT 1, 6
• Women with a history of GDM have an increased risk of developing type 2 diabetes and should receive lifelong screening at least every 3 years 1, 6, 7
• Lifestyle modifications, breastfeeding, and in some cases metformin can reduce the risk of developing type 2 diabetes after GDM 6, 7