Screening for Gestational Diabetes Mellitus in Pregnancy
All pregnant women should be screened for gestational diabetes mellitus (GDM) at 24-28 weeks of gestation using either a one-step 75g oral glucose tolerance test (OGTT) or a two-step approach with a 50g glucose challenge test followed by a 100g OGTT if the initial screen is positive. 1, 2, 3
Standard Screening Protocol
Timing of Screening
- Universal screening should occur between 24-28 weeks of gestation for all pregnant women not previously diagnosed with diabetes 1, 3
- This timing is based on physiological changes in glucose metabolism that occur during the second half of pregnancy, when insulin resistance peaks 4
- Women at very high risk should undergo early screening at the first prenatal visit (12-14 weeks) and be rescreened at 24-28 weeks if initially negative 1, 2
High-Risk Criteria Requiring Early Screening
Women meeting any of the following criteria should be screened at their first prenatal visit:
- BMI ≥30 kg/m² (most important modifiable risk factor) 2
- Prior history of GDM or delivery of a large-for-gestational-age infant 1, 2
- Family history of diabetes in first-degree relatives 1, 2
- High-risk ethnicity (Hispanic, Native American, South or East Asian, African American, or Pacific Island descent) 1, 2
- Polycystic ovary syndrome (PCOS) 1, 2
- Presence of glycosuria 1
Screening Methods
Two-Step Approach (Most Common in United States)
Step 1: Initial Screening
- Administer 50g oral glucose load in a non-fasting state 1, 2
- Measure plasma glucose at 1 hour 1, 3
- Threshold of ≥140 mg/dL identifies 80% of women with GDM 1
- Alternative threshold of ≥130 mg/dL increases sensitivity to 90% but reduces specificity 1
Step 2: Diagnostic Testing (if Step 1 positive)
- Perform 100g OGTT after overnight fast of at least 8 hours 1
- Measure plasma glucose at fasting, 1 hour, 2 hours, and 3 hours 1
- Diagnosis requires at least two of the following values (Carpenter/Coustan criteria):
One-Step Approach (IADPSG Consensus)
- Perform 75g OGTT after overnight fast of at least 8 hours at 24-28 weeks 1, 2
- Measure plasma glucose at fasting, 1 hour, and 2 hours 1
- Diagnosis requires only ONE of the following values to be met or exceeded:
Important Clinical Considerations
Early Pregnancy Screening (Before 24 Weeks)
- For high-risk women screened early, use standard diabetes diagnostic criteria rather than GDM criteria 1, 2
- If fasting plasma glucose ≥126 mg/dL or random glucose ≥200 mg/dL with symptoms, diagnose as overt diabetes, not GDM 2
- All women with negative early screening must be rescreened at 24-28 weeks 2, 5
Women with Prior GDM
- Screen as early as possible at first prenatal visit (typically 12-14 weeks) 2
- If negative, mandatory repeat screening at 24-28 weeks 2, 5
- These women have a 4.14 times higher risk of recurrent GDM 2
Postpartum Follow-Up
- All women diagnosed with GDM should undergo 75g OGTT at 4-12 weeks postpartum using non-pregnant diagnostic criteria 1, 2, 3
- Lifelong screening for diabetes or prediabetes should continue at least every 3 years 2, 5
- Women with prediabetes should receive intensive lifestyle interventions or metformin 2
Critical Pitfalls to Avoid
- Failing to rescreen high-risk women at 24-28 weeks after negative early screening leads to missed diagnoses, as 38.7% of GDM develops before 24 weeks but glucose intolerance continues to evolve 2, 6
- Screening only women with risk factors misses 42% of GDM cases that occur in women without identifiable risk factors 7
- Using only fasting glucose for screening has poor sensitivity compared to post-load glucose values 1
- Forgetting postpartum screening leaves women at high risk for type 2 diabetes without appropriate surveillance 2, 5