When should a pregnant woman with a history of gestational diabetes mellitus (GDM) undergo a Fasting Blood Sugar (FBS) test versus a 75g Oral Glucose Tolerance Test (OGTT)?

When to Use FBS vs 75g OGTT in Pregnant Women

For pregnant women with a history of GDM, perform early screening with FBS (or any glucose test) at the first prenatal visit (12-14 weeks), and if negative, mandatory 75g OGTT screening at 24-28 weeks. 1, 2

Early Pregnancy Screening (First Prenatal Visit)

Who Needs Early Screening

Women with the following risk factors should be tested at their first prenatal visit (typically 12-14 weeks):

• History of previous GDM (4.14 times higher risk) 2
• BMI ≥30 kg/m² 2
• Family history of diabetes in first-degree relatives 2
• High-risk ethnicity (Hispanic, Native American, South or East Asian, African American, Pacific Islander) 2
• History of delivering macrosomic baby (>4.05 kg) 2
• History of polycystic ovary syndrome 2

What Test to Use Early

At the first prenatal visit, use FBS or random plasma glucose to detect overt (pre-existing) diabetes, not GDM: 1, 3

• FBS ≥126 mg/dL = overt diabetes (not GDM) 1
• Random glucose ≥200 mg/dL with symptoms = overt diabetes 1
• FBS 92-125 mg/dL = diagnose as GDM 1
• FBS <92 mg/dL = proceed to 75g OGTT at 24-28 weeks 1

Critical point: Early screening is designed to catch pre-existing undiagnosed type 2 diabetes, not true GDM which develops later in pregnancy. 2 The 75g OGTT is not routinely recommended before 24 weeks even in high-risk women. 1

Standard Screening at 24-28 Weeks (Mandatory for All)

Universal Screening Window

All pregnant women not previously diagnosed with overt diabetes or GDM must undergo screening at 24-28 weeks, including those who tested negative early. 1, 2, 4

Two Screening Approaches

One-Step Approach (75g OGTT):

• Perform fasting 75g OGTT with measurements at fasting, 1-hour, and 2-hour 1, 4
• Diagnosis requires only ONE abnormal value: 1, 4, 5
  • Fasting ≥92 mg/dL
  • 1-hour ≥180 mg/dL
  • 2-hour ≥153 mg/dL

Two-Step Approach (50g GCT → 100g OGTT):

• Initial 50g glucose challenge test (non-fasting) 1, 2
• If 1-hour glucose ≥140 mg/dL, proceed to fasting 100g OGTT 1
• Diagnosis requires at least TWO abnormal values: 1, 4
  • Fasting ≥95 mg/dL
  • 1-hour ≥180 mg/dL
  • 2-hour ≥155 mg/dL
  • 3-hour ≥140 mg/dL

The American College of Obstetricians and Gynecologists currently supports the two-step approach, while the American Diabetes Association endorses the one-step approach. 4

Postpartum Follow-Up

Immediate Postpartum Testing

All women diagnosed with GDM must undergo 75g OGTT at 4-12 weeks postpartum using non-pregnancy diagnostic criteria. 1, 4, 5

Why 75g OGTT and not FBS or A1C postpartum?

• A1C may be falsely lowered by increased red blood cell turnover during pregnancy, blood loss at delivery, or the preceding glucose profile 1
• The 75g OGTT is more sensitive at detecting glucose intolerance including both prediabetes and diabetes 1

Long-Term Surveillance

Women with history of GDM require lifelong screening for diabetes at least every 1-3 years using any glycemic test (A1C, FBS, or 75g OGTT). 1, 2, 4, 5

Common Pitfalls to Avoid

Not repeating screening at 24-28 weeks in high-risk women who initially test negative leads to delayed diagnosis and treatment. 2 Even with normal early screening, GDM typically develops in the second/third trimester due to increasing insulin resistance.

Using A1C for postpartum diagnosis is unreliable due to pregnancy-related changes in red blood cell turnover. 1 Always use 75g OGTT at 4-12 weeks postpartum.

Performing 75g OGTT before 24 weeks routinely is not recommended even in high-risk women, as there is insufficient evidence of benefit. 1 Use FBS or random glucose for early screening instead.

Failing to screen high-risk women early may miss pre-existing undiagnosed type 2 diabetes, particularly in women with BMI ≥30 kg/m². 2