What are the recommended screening, diagnostic criteria, and management guidelines for gestational diabetes mellitus?

Gestational Diabetes Mellitus: Screening, Diagnosis, and Management

Screening Strategy

All pregnant women without known diabetes should be screened for gestational diabetes mellitus (GDM) at 24-28 weeks of gestation, with high-risk women requiring additional early screening at their first prenatal visit. 1, 2

Risk-Based Early Screening (First Prenatal Visit)

Women with the following high-risk characteristics should undergo glucose testing as soon as feasible at their first prenatal visit 1:

• Marked obesity (BMI ≥30 kg/m²) 3, 4
• Personal history of GDM 3, 4
• Strong family history of diabetes (first-degree relatives) 1, 3
• Glycosuria 1
• High-risk ethnicity (Hispanic American, Native American, Asian American, African-American, Pacific Islander) 1, 3

If early screening is negative in high-risk women, mandatory repeat testing at 24-28 weeks is required because insulin resistance increases exponentially in the second and third trimesters 3, 4.

Low-Risk Women Who May Skip Screening

Women meeting all of the following criteria may be excluded from routine screening 1:

• Age younger than 25 years
• BMI ≤25 kg/m² with normal pre-pregnancy weight
• No first-degree relative with diabetes
• No history of abnormal glucose metabolism
• No history of poor obstetric outcomes
• Member of ethnic group with low diabetes prevalence

However, universal screening at 24-28 weeks is now considered standard practice in the United States, as the vast majority of pregnant women have at least one risk factor 1, 2.

Diagnostic Approaches

Two acceptable strategies exist for GDM diagnosis, with regional preferences determining which is used 1:

Two-Step Approach (Commonly Used in U.S.)

Step 1: 50g glucose challenge test (GCT), non-fasting 1, 3

• If plasma glucose ≥130-140 mg/dL at 1 hour, proceed to Step 2
• The threshold (130 vs 135 vs 140 mg/dL) is set by local consensus 1

Step 2: 100g oral glucose tolerance test (OGTT), fasting 1, 3

• Diagnosis requires ≥2 abnormal values (Carpenter-Coustan criteria) 1:
  • 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)

One-Step Approach (IADPSG/WHO Criteria)

75g OGTT, fasting, performed at 24-28 weeks 1, 2

• Diagnosis requires only 1 abnormal value 1, 2:
  • 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)

The one-step approach diagnoses approximately twice as many women with GDM compared to the two-step approach, though evidence on differential outcomes remains mixed 1. The one-step criteria are based on the landmark HAPO study, which demonstrated continuous relationships between maternal glucose levels and adverse outcomes including macrosomia, cesarean delivery, neonatal hypoglycemia, and fetal hyperinsulinism 1.

Testing Conditions

For accurate OGTT results 1, 4:

• Perform after 8-14 hour overnight fast
• Ensure ≥3 days of unrestricted diet (≥150g carbohydrate daily)
• Patient remains seated throughout test
• No smoking during test

Early Pregnancy Detection of Pre-existing Diabetes

At the first prenatal visit in high-risk women, the following values indicate overt diabetes (not GDM) and require immediate management 1, 3:

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), confirmed on subsequent day
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms
• A1C ≥6.5% (though A1C is not recommended for routine GDM screening due to lack of sensitivity) 2, 3

Management Principles

Lifestyle Modifications (First-Line)

70-85% of women with GDM achieve glycemic control with lifestyle modifications alone 2, 5:

• Medical nutrition therapy: Calorie restriction with low glycemic index diet to prevent postprandial hyperglycemia 6, 5
• Regular physical activity 6, 5
• Self-monitoring of blood glucose 3-4 times daily (fasting and postprandial) 1, 7

Glycemic Targets

Target glucose levels during pregnancy 7, 5:

• Fasting: <95 mg/dL
• 1-hour postprandial: <140 mg/dL
• 2-hour postprandial: <120 mg/dL

Pharmacological Therapy

Insulin remains the first-line pharmacological treatment when lifestyle modifications fail to achieve target glucose levels 7, 5. Metformin is increasingly used as an alternative when insulin cannot be prescribed, though up to 46% of women on metformin may require supplemental insulin 5.

Fetal Surveillance

Women with GDM require enhanced antenatal surveillance including ultrasound monitoring for fetal growth and amniotic fluid volume 7. Treatment of GDM significantly reduces risks of macrosomia, neonatal hypoglycemia, shoulder dystocia, and cesarean delivery 2, 4.

Postpartum and Long-Term Follow-Up

Immediate Postpartum Screening

All women with GDM must be screened for persistent diabetes at 4-12 weeks postpartum using a 75g OGTT with non-pregnancy diagnostic criteria (not A1C) 1, 3. This is critical because the majority of GDM cases resolve after delivery, but reclassification is essential 1.

Lifelong Surveillance

Women with history of GDM have 50-60% lifetime risk of developing type 2 diabetes 2, 8 and should receive 1, 3:

• Lifelong screening for diabetes or prediabetes at least every 3 years 1, 3
• Intensive lifestyle interventions or metformin if prediabetes is detected 1, 3
• Counseling about increased risks of cardiovascular disease, hypertensive disorders, and metabolic syndrome 5

Subsequent Pregnancies

Women with prior GDM should undergo glucose testing as early as possible at the first prenatal visit in subsequent pregnancies, and if negative, mandatory retesting at 24-28 weeks 3, 4. Preconception screening before future pregnancies is universally recommended 7.

Common Pitfalls to Avoid

• Do not rely on fasting glucose alone for GDM diagnosis—a full OGTT is required as fasting glucose may miss postprandial hyperglycemia, the primary driver of macrosomia 3, 4
• Do not skip 24-28 week rescreening in high-risk women with negative early screening, as this leads to delayed diagnosis and increased complications 3, 4
• Do not use A1C for routine GDM screening during pregnancy due to insufficient sensitivity 2, 3
• Do not screen average-risk patients before 24 weeks based solely on non-first-degree relative family history 2, 3
• Do not delay screening in obese women (BMI ≥30)—test immediately at first prenatal visit 4