What are the diagnostic criteria and management steps for a pregnant woman screened for gestational diabetes mellitus?

Gestational Diabetes Mellitus: Diagnosis and Management

Diagnostic Approach

All pregnant women without pre-existing diabetes should undergo universal screening for gestational diabetes at 24–28 weeks gestation using either a one-step 75g oral glucose tolerance test or a two-step approach, with high-risk women requiring additional early screening at the first prenatal visit. 1

Timing of Screening

Standard Universal Screening (24–28 weeks):

• This window corresponds to peak pregnancy-related insulin resistance when GDM most commonly manifests and when diagnostic thresholds have been validated 1
• The HAPO study demonstrated continuous associations between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) specifically at this gestational age 1
• Do not postpone screening beyond 28 weeks—delayed diagnosis increases rates of preeclampsia, macrosomia, shoulder dystocia, and neonatal hypoglycemia 1

Early Screening (First Prenatal Visit, 12–14 weeks):

High-risk women require immediate glucose testing to detect undiagnosed pre-existing type 2 diabetes 1, 2:

• BMI ≥30 kg/m² (single most important risk factor) 1, 2
• Prior history of gestational diabetes 1, 2, 3
• First-degree relative with diabetes 1, 2
• Glycosuria on routine urinalysis 1, 2
• High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 1

Critical pitfall: If early screening is negative in high-risk women, mandatory repeat testing at 24–28 weeks is required because insulin resistance rises exponentially in later trimesters 1, 2, 3

Diagnostic Criteria

One-Step Approach (IADPSG/ADA Criteria):

Perform a 75g OGTT after 8–14 hour overnight fast with plasma glucose measured at fasting, 1-hour, and 2-hour 1:

• Fasting ≥92 mg/dL (5.1 mmol/L) 1
• 1-hour ≥180 mg/dL (10.0 mmol/L) 1
• 2-hour ≥153 mg/dL (8.5 mmol/L) 1

GDM is diagnosed when ANY ONE value is met or exceeded 1

This approach identifies approximately 15–20% of pregnancies with GDM 1

Two-Step Approach (ACOG-Supported):

Step 1: 50g non-fasting glucose challenge test at 24–28 weeks 1, 2

• If 1-hour plasma glucose ≥130–140 mg/dL, proceed to Step 2 1, 2

Step 2: 100g OGTT after overnight fast with measurements at fasting, 1,2, and 3 hours 1, 2

Carpenter-Coustan criteria—GDM diagnosed when AT LEAST TWO values are met or exceeded 1, 2:

• Fasting ≥95 mg/dL (5.3 mmol/L) 1, 2
• 1-hour ≥180 mg/dL (10.0 mmol/L) 1, 2
• 2-hour ≥155 mg/dL (8.6 mmol/L) 1, 2
• 3-hour ≥140 mg/dL (7.8 mmol/L) 1, 2

ACOG notes that in clinical practice a single elevated value may be used for diagnosis, though traditional criteria require two abnormal values 1

This approach identifies approximately 5–6% of pregnancies with GDM 1

Test Preparation Requirements

For accurate OGTT results, patients must 1:

• Fast overnight for 8–14 hours 1
• Consume ≥150g carbohydrate daily for at least 3 days before testing 1
• Maintain unrestricted physical activity in preceding days 1
• Remain seated throughout the test 1
• Avoid smoking during testing 1

Early Pregnancy Diagnostic Thresholds for Overt Diabetes

When screening high-risk women at first prenatal visit, use non-pregnancy diagnostic criteria 1, 2:

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) indicates overt pre-existing diabetes 1, 2
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms indicates overt diabetes 1, 2
• Early-pregnancy fasting glucose ≥92 mg/dL but <126 mg/dL meets GDM diagnostic criteria 1

Critical pitfall: Do not apply the gestational diabetes thresholds (92/180/153 mg/dL) to early-pregnancy screening—these were derived from 24–28 week data 1

Tests NOT Recommended for GDM Screening

• Hemoglobin A1C is not recommended for GDM screening due to poor sensitivity and specificity 1, 2
• A1C ≥6.5% may identify pre-existing diabetes early in pregnancy but should not be used for routine GDM screening 1, 2
• Random plasma glucose measurements are not validated for GDM screening 1
• Urine glucose testing is not useful for GDM management and should be avoided 1

Low-Risk Women Who May Skip Screening

Women meeting ALL of the following criteria may forgo GDM screening 1, 2:

• Age <25 years 1, 2
• Pre-pregnancy BMI ≤25 kg/m² 1, 2
• No first-degree relative with diabetes 1, 2
• No prior abnormal glucose tolerance 1, 2
• No history of adverse obstetric outcomes 1, 2
• Ethnic group with low diabetes prevalence 1, 2

In practice, most pregnant women require screening because these criteria are stringent 2

Management After Diagnosis

Fetal Surveillance

• Ultrasound monitoring of fetal abdominal circumference is recommended 1
• Measurement exceeding the 75th percentile for gestational age suggests fetal hyperinsulinemia and warrants intensified maternal glycemic control 1

Treatment Approach

• Initiate dietary modification, regular physical activity, and glucose self-monitoring 1
• Add pharmacologic therapy (insulin or metformin) when lifestyle measures are insufficient 1

Postpartum Follow-Up

All women diagnosed with GDM must undergo 1, 2, 3:

• 75g OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes 1, 2, 3
• The 75g OGTT is preferred over fasting glucose or A1C due to higher sensitivity for detecting glucose intolerance 3

Long-term surveillance 1, 2, 3:

• Lifelong diabetes screening every 3 years (some guidelines recommend every 1–3 years) 1, 2, 3
• Women with prior GDM have a 3.4-fold increased risk of developing type 2 diabetes 1
• If postpartum prediabetes is identified, initiate intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes 1, 2

Choosing Between One-Step and Two-Step Approaches

The one-step 75g OGTT is more sensitive and identifies twice as many women with GDM compared to the two-step approach 1, 2:

• In high-risk populations, the one-step strategy is more cost-effective and identifies women with milder hyperglycemia who still benefit from treatment 1
• The two-step approach reduces the number of women requiring full diagnostic OGTT but may miss milder cases 1
• Both strategies are acceptable; choice depends on practice setting, GDM prevalence in the population, and available resources 1
• Treatment of GDM reduces serious perinatal complications with a number needed to treat of 34 to prevent macrosomia and shoulder dystocia 2