What is the diagnostic workup and management for a pregnant woman suspected of having gestational diabetes?

Gestational Diabetes Diagnosis

Screen all pregnant women not previously known to have diabetes at 24-28 weeks of gestation using either a one-step or two-step approach, with the two-step method being the preferred standard in most U.S. practices. 1, 2, 3

Early Screening for High-Risk Women

Test women with risk factors for undiagnosed type 2 diabetes at the first prenatal visit (ideally first trimester, up to 12 weeks) using standard non-pregnancy diagnostic criteria, not gestational diabetes criteria. 1, 4, 2, 3

Risk Factors Requiring Early Screening:

• BMI ≥30 kg/m² (this patient qualifies with BMI 35) 4, 3
• Previous history of gestational diabetes 3
• Family history of diabetes 3
• High-risk ethnicity (non-Hispanic White has lower risk) 1
• History of delivering a macrosomic infant 3
• Polycystic ovary syndrome 3

Critical Point for This Patient:

At 16 weeks gestation with BMI 35, this patient should be tested immediately if not already screened at the first prenatal visit, then rescreened at 24-28 weeks if the initial test is negative. 4, 2, 3 The rationale is that women with obesity have significantly increased prevalence of undiagnosed type 2 diabetes (approximately 4.5% of reproductive-age women have diabetes, with 30% unaware), and early detection allows prompt intervention to reduce complications including the 20% risk of macrosomia with untreated gestational diabetes. 1, 4

Diagnostic Testing Approaches

Two-Step Approach (ACOG-Endorsed):

Step 1: Initial Screening

• Perform 50g glucose challenge test (GCT) in non-fasting state 4, 3
• Measure plasma glucose at 1 hour 3
• Threshold: ≥130-140 mg/dL is positive and requires diagnostic testing 4, 3

Step 2: Diagnostic Confirmation (if GCT positive)

• Perform 100g oral glucose tolerance test (OGTT) after 8-14 hour overnight fast 4, 3
• Measure plasma glucose at fasting, 1 hour, 2 hours, and 3 hours 3
• Diagnosis requires ≥2 of the following values (Carpenter-Coustan criteria): 2, 3
  • Fasting ≥95 mg/dL
  • 1 hour ≥180 mg/dL
  • 2 hours ≥155 mg/dL
  • 3 hours ≥140 mg/dL

One-Step Approach (IADPSG-Endorsed):

Perform 75g OGTT directly after 8-14 hour overnight fast 4, 2, 3

• Measure plasma glucose at fasting, 1 hour, and 2 hours 2, 3
• Diagnosis requires only ONE elevated value: 2, 3
  • Fasting ≥92 mg/dL
  • 1 hour ≥180 mg/dL
  • 2 hours ≥153 mg/dL

Choosing Between Approaches:

The two-step approach is currently supported by ACOG and remains the standard in most U.S. practices, while the one-step approach is recommended by the International Association of Diabetes and Pregnancy Study Groups. 1, 2, 3 The one-step approach more than doubles the incidence of gestational diabetes (from 5-6% to 15-20%) but its clinical benefit in reducing adverse pregnancy outcomes remains controversial. 1, 5 For high-risk populations like this patient with BMI 35, the one-step 75g OGTT may be more cost-effective by eliminating the screening step. 4

Critical Testing Requirements

Ensure proper test conditions: 4

• 8-14 hour overnight fast
• At least 3 days of unrestricted diet (≥150g carbohydrate daily)
• Patient remains seated and does not smoke during testing

Common Pitfalls to Avoid

Do NOT rely on fasting glucose alone for diagnosis—a normal fasting glucose does not rule out gestational diabetes and a full OGTT is required for definitive diagnosis. 4, 2 This is a critical error that leads to missed diagnoses.

Do NOT skip the mandatory 24-28 week rescreen if early testing is negative. 4, 3 Insulin resistance increases exponentially in the second and third trimesters, and failing to rescreen high-risk women who initially test negative leads to delayed diagnosis and increased maternal-fetal complications. 4

Do NOT delay screening—this patient at 16 weeks should be tested immediately. 4

Postpartum and Long-Term Follow-Up

Test all women with gestational diabetes for persistent diabetes or prediabetes at 4-12 weeks postpartum using a 75g OGTT with non-pregnancy diagnostic criteria. 1, 2, 3, 6

Women with a history of gestational diabetes have 3.4 times higher risk of developing type 2 diabetes and require lifelong screening for diabetes or prediabetes at least every 3 years. 1, 2, 3

Women found to have prediabetes should receive intensive lifestyle interventions or metformin to prevent diabetes. 1, 3

Clinical Rationale for This Patient

With BMI 35, this patient faces compounded risks: obesity independently increases fetal macrosomia risk beyond diabetes effects, and undiagnosed gestational diabetes in obese women carries macrosomia rates up to 20%. 4 Early screening at this stage (16 weeks) is intended to detect pre-existing undiagnosed type 2 diabetes that was present before pregnancy, which requires more intensive management from the outset and is associated with increased risk of congenital malformations if present at conception. 1, 2 Treatment of screen-detected gestational diabetes with dietary modifications, glucose monitoring, and insulin (if needed) significantly reduces the risk for preeclampsia, fetal macrosomia, and shoulder dystocia. 4