What are the American College of Obstetricians and Gynecologists (ACOG) guidelines for diagnosing gestational diabetes?

ACOG Guidelines for Diagnosing Gestational Diabetes

ACOG recommends risk-based screening with universal testing at 24-28 weeks' gestation using a two-step approach: a 50-g glucose challenge test followed by a 100-g oral glucose tolerance test if the initial screen is abnormal. 1

Risk Assessment and Early Screening

High-risk women should undergo glucose testing at the first prenatal visit, with retesting at 24-28 weeks if initial results are negative. 1 High-risk characteristics include:

• Marked obesity (BMI ≥30 kg/m²) 2
• Personal history of gestational diabetes 1, 2
• Glycosuria 1
• Strong family history of diabetes in first-degree relatives 1, 2
• High-risk ethnicity (Hispanic, Native American, South or East Asian, African American, or Pacific Islander descent) 2
• History of delivering a macrosomic infant (>4.05 kg or 9 lb) 2
• Polycystic ovary syndrome 2

Low-risk women may be exempted from screening if they meet ALL of the following criteria: age <25 years, pre-pregnancy BMI ≤25 kg/m², no first-degree relative with diabetes, no prior abnormal glucose metabolism, and belonging to an ethnic group with low diabetes prevalence. 2 However, most women require testing based on these inclusive criteria. 1

Standard Screening Protocol (Two-Step Approach)

Step 1: Glucose Challenge Test (GCT)

• Administer 50-g oral glucose load (non-fasting) at 24-28 weeks' gestation 1, 2
• Measure plasma glucose at 1 hour 2
• Threshold for abnormal result: ≥140 mg/dL (some use ≥130 mg/dL) 1, 2
• If abnormal, proceed to Step 2 1, 2

Step 2: Diagnostic Oral Glucose Tolerance Test (OGTT)

• Administer 100-g oral glucose load after 8-14 hour overnight fast 2, 3
• Measure plasma glucose at four time points: fasting, 1 hour, 2 hours, and 3 hours 2, 3
• Diagnosis requires ≥2 abnormal values (Carpenter-Coustan criteria): 2, 3
  • Fasting ≥95 mg/dL (5.3 mmol/L)
  • 1 hour ≥180 mg/dL (10.0 mmol/L)
  • 2 hours ≥155 mg/dL (8.6 mmol/L)
  • 3 hours ≥140 mg/dL (7.8 mmol/L)

ACOG currently supports the two-step approach but notes that a single elevated value may be used for diagnosis. 3

Alternative One-Step Approach

While ACOG traditionally favors the two-step method, the one-step approach using a 75-g OGTT is also recognized: 2

• Perform after 8-14 hour overnight fast at 24-28 weeks 2
• Measure plasma glucose at three time points: fasting, 1 hour, and 2 hours 2
• Diagnosis requires only 1 abnormal value (IADPSG criteria): 2
  • Fasting ≥92 mg/dL (5.1 mmol/L)
  • 1 hour ≥180 mg/dL (10.0 mmol/L)
  • 2 hours ≥153 mg/dL (8.5 mmol/L)

The one-step approach identifies approximately 2-3 times more cases of GDM (15-20% prevalence) compared to the two-step approach (5-6% prevalence). 3 The choice between approaches remains controversial and may depend on population prevalence, resources, and cost-effectiveness considerations. 3

Early Pregnancy Detection of Pre-Existing Diabetes

At the first prenatal visit in high-risk women, the following thresholds indicate overt pre-existing diabetes (not GDM): 2

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), confirmed on repeat testing
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic hyperglycemia symptoms
• HbA1c ≥6.5% (though A1C is not recommended for routine GDM screening due to limited sensitivity) 2

Critical Testing Conditions

For accurate OGTT results, patients must: 2

• Observe an 8-14 hour overnight fast
• Consume ≥150 g carbohydrate daily for at least 3 days before testing

Postpartum and Long-Term Follow-Up

All women diagnosed with GDM must undergo a 75-g OGTT at 4-12 weeks postpartum using non-pregnancy diagnostic criteria to identify persistent diabetes. 2, 3 This is critical because most GDM resolves after delivery, but a notable proportion progresses to overt diabetes. 2

Lifelong surveillance is mandatory: Women with prior GDM should be screened for diabetes or prediabetes at least every 3 years throughout adulthood. 2 If prediabetes is detected, intensive lifestyle intervention or metformin therapy should be initiated to prevent progression to type 2 diabetes. 2

Common Pitfalls to Avoid

• Failing to retest high-risk women at 24-28 weeks after negative early screening may result in delayed diagnosis, as insulin resistance increases exponentially in the second and third trimesters. 2
• Not performing postpartum screening misses the opportunity to identify persistent diabetes and initiate preventive measures. 2
• Overlooking the need for lifelong screening in women with prior GDM, who have a 3.4-fold increased risk of developing type 2 diabetes. 4

Clinical Rationale

Treatment of GDM reduces serious perinatal complications with a number needed to treat of 34, specifically reducing macrosomia and shoulder dystocia. 1 Untreated gestational diabetes is associated with higher incidence of these complications. 1 The HAPO study demonstrated a continuous relationship between maternal glucose levels at 24-28 weeks and adverse outcomes, with no clear threshold for risk. 2