When should a 75‑gram oral glucose tolerance test be performed for gestational diabetes screening in pregnancy, including universal screening at 24‑28 weeks and earlier testing for high‑risk factors?

When to Request 75g OGTT in Pregnancy

All pregnant women without known pre-existing diabetes should undergo universal screening for gestational diabetes at 24–28 weeks of gestation using a 75g oral glucose tolerance test (OGTT), and high-risk women should receive additional early screening at their first prenatal visit. 1, 2

Universal Screening at 24–28 Weeks

• Perform a 75g OGTT at 24–28 weeks gestation in all pregnant women not previously diagnosed with diabetes, as this represents the period of maximal insulin resistance when gestational diabetes typically manifests. 1, 2

• The test must be performed in the morning after an overnight fast of at least 8 hours, with plasma glucose measurements taken at fasting, 1 hour, and 2 hours after the glucose load. 1, 3

• Diagnosis requires only ONE abnormal value (one-step approach per IADPSG/WHO criteria):

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

• This universal screening recommendation is based on the landmark HAPO study, which demonstrated a continuous relationship between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) with no clear threshold for risk. 1, 2

Early Screening at First Prenatal Visit (12–14 Weeks)

High-risk women require early screening to detect pre-existing undiagnosed type 2 diabetes that predates pregnancy. 1, 2

High-Risk Criteria Requiring Early Screening:

• BMI ≥30 kg/m² – the single most important risk factor warranting early testing 1, 2
• History of prior gestational diabetes – confers a 4.14-fold increased risk 2
• First-degree relative with diabetes (parent or sibling) 1, 2
• High-risk ethnicity: Hispanic, Native American, South or East Asian, African American, or Pacific Islander descent 1, 2
• History of delivering a macrosomic infant (>4.05 kg or 9 lb) 2
• Polycystic ovary syndrome (PCOS) 2
• Glycosuria on routine urinalysis 2

Early Screening Protocol:

• At the first prenatal visit (typically 12–14 weeks), use standard diagnostic criteria for overt diabetes:

  • Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) indicates pre-existing diabetes 1
  • Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with hyperglycemia symptoms indicates pre-existing diabetes 1
  • HbA1c ≥6.5% indicates pre-existing diabetes (though A1C has limited sensitivity for GDM screening) 1

• If early screening is negative, mandatory repeat screening at 24–28 weeks is required because insulin resistance increases exponentially in the second and third trimesters. 1, 2

Women Who May Skip Early Screening

• Women with BMI <25 kg/m² who meet ALL of the following low-risk criteria may proceed directly to 24–28 week screening:

  • Age <25 years 2
  • No first-degree relative with diabetes 2
  • No history of abnormal glucose tolerance 2
  • No history of adverse obstetric outcomes 2
  • Not belonging to a high-risk ethnic group 2

• In practice, most pregnant women require screening because these exemption criteria are extremely stringent and rarely all met simultaneously. 2

Alternative Two-Step Approach

While the one-step 75g OGTT is increasingly preferred internationally, the American College of Obstetricians and Gynecologists continues to support a two-step approach:

• Step 1: 50g glucose challenge test (non-fasting) at 24–28 weeks; if plasma glucose ≥130–140 mg/dL at 1 hour, proceed to Step 2 1

• Step 2: 100g OGTT (fasting); diagnosis requires at least 2 abnormal values:

  • Fasting ≥95 mg/dL 1
  • 1-hour ≥180 mg/dL 1
  • 2-hour ≥155 mg/dL 1
  • 3-hour ≥140 mg/dL 1

• The one-step 75g OGTT identifies approximately 2–3 times more cases of GDM compared to the two-step approach (15–20% vs. 5–6% prevalence), though this remains controversial. 4

Critical Pitfalls to Avoid

• Failing to repeat screening at 24–28 weeks in high-risk women with negative early results is the most common error, as GDM typically develops later in pregnancy when insulin resistance peaks. 1, 2

• Using GDM diagnostic criteria (92/180/153 mg/dL) for early pregnancy screening is inappropriate; early screening is intended to detect pre-existing diabetes using standard non-pregnancy thresholds (≥126 mg/dL fasting). 1

• Relying on early screening alone in obese women without 24–28 week follow-up will miss the majority of GDM cases. 2

• Research shows that early OGTT at 14–16 weeks has poor sensitivity for predicting subsequent GDM in low-risk populations and should not replace standard 24–28 week screening. 5

Postpartum and Long-Term Follow-Up

• All women diagnosed with GDM must undergo a 75g OGTT at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to identify persistent diabetes or prediabetes. 1, 3

• Lifelong screening every 3 years is mandatory for women with a history of GDM, as they have a 3.4-fold increased risk of developing type 2 diabetes. 1, 3

• Women found to have prediabetes postpartum should receive intensive lifestyle interventions or metformin therapy to prevent progression to overt diabetes. 1, 3