What are the diagnostic criteria and management plan for a pregnant woman undergoing a 75g Oral Glucose Tolerance Test (OGTT)?

How to Interpret the 75g OGTT in Pregnancy

Diagnose gestational diabetes mellitus (GDM) if any single value meets or exceeds these thresholds: fasting ≥92 mg/dL (5.1 mmol/L), 1-hour ≥180 mg/dL (10.0 mmol/L), or 2-hour ≥153 mg/dL (8.5 mmol/L). 1, 2, 3

Test Administration Requirements

The 75g OGTT must be performed under specific conditions to ensure accurate interpretation:

• Timing: Conduct between 24-28 weeks of gestation in women without previously diagnosed diabetes 1, 2
• Fasting requirement: Patient must fast overnight for at least 8 hours before the test 1, 4
• Morning administration: Perform the test in the morning 4
• Measurement intervals: Draw plasma glucose at three time points: fasting (before glucose load), 1 hour after, and 2 hours after the 75g glucose load 1, 4

Diagnostic Criteria (One-Step Approach)

The diagnosis requires only ONE elevated value, not multiple abnormal results. 1, 2, 3 This represents a critical departure from older criteria that required two abnormal values. The specific thresholds are:

• Fasting: ≥92 mg/dL (5.1 mmol/L) 1, 2, 3
• 1-hour: ≥180 mg/dL (10.0 mmol/L) 1, 2, 3
• 2-hour: ≥153 mg/dL (8.5 mmol/L) 1, 2, 3

These thresholds are based on the landmark HAPO study, which demonstrated continuous associations between maternal glucose levels and adverse pregnancy outcomes including large-for-gestational-age births, cesarean delivery, neonatal hypoglycemia, and preeclampsia. 1 The cutoffs represent glucose levels associated with an odds ratio of 1.75 for adverse outcomes compared to mean glucose levels. 1

Clinical Context and Risk Stratification

High-risk women should undergo early screening at the first prenatal visit using fasting plasma glucose, and if normal, repeat testing at 24-28 weeks. 2, 5 High-risk characteristics include:

• History of GDM or prediabetes 5
• Previous macrosomic infant (>4500g), stillbirth, or recurrent pregnancy loss 5
• Obesity or metabolic syndrome 5
• Age >35 years 5
• Vascular disease 5
• High-risk ethnicity (Arab, South/Southeast Asian, Latin American) 5

Understanding the Prevalence Impact

This one-step approach identifies approximately 15-20% of pregnant women with GDM, compared to 5-6% with older two-step criteria. 1, 4 While this represents a substantial increase in diagnoses, it reflects the continuous relationship between maternal glucose and adverse outcomes—there is no clear threshold below which risk disappears. 1

The American Diabetes Association adopted these IADPSG criteria in 2011, prioritizing pregnancy outcomes over subsequent maternal diabetes risk. 1 The American College of Obstetricians and Gynecologists acknowledges that a single elevated value can establish the diagnosis. 2, 3

Immediate Management After Positive Test

Once GDM is diagnosed:

• Initiate nutritional counseling immediately for all women 3
• Establish glucose monitoring targets: fasting <95 mg/dL, 1-hour postprandial <140 mg/dL, 2-hour postprandial <120 mg/dL 3
• Begin lifestyle modifications first: 70-85% of women achieve adequate control with diet and exercise alone 3
• Add insulin therapy if glucose levels remain above target despite lifestyle modifications 3, 5
• Implement regular obstetric monitoring including ultrasound examinations 3, 5

Critical Postpartum Follow-Up

All women with GDM must undergo reevaluation 4-12 weeks postpartum with a 75g OGTT to identify persistent diabetes or prediabetes. 2, 3, 5 This is non-negotiable, as these women have a 3.4-fold increased risk of developing type 2 diabetes. 2, 3

Lifelong screening is required at least every 3 years for women with GDM history. 2, 3 Those found to have prediabetes should receive intensive lifestyle interventions or metformin for diabetes prevention. 2

Common Pitfalls to Avoid

• Do not require two abnormal values: Unlike older criteria, the current standard diagnoses GDM with any single elevated measurement 1
• Do not perform OGTT after bariatric surgery: Risk of postprandial hypoglycemia makes this test inappropriate in this population 5
• Do not skip postpartum testing: Many clinicians fail to ensure follow-up glucose testing occurs, missing opportunities for early diabetes intervention 2, 3
• Do not use HbA1c alone for GDM diagnosis: The 75g OGTT remains the gold standard for gestational diabetes screening 5