At what Body Mass Index (BMI) should a pregnant woman be screened for Diabetes Mellitus (DM)?

BMI Threshold for Gestational Diabetes Screening

Pregnant women with a BMI ≥30 kg/m² should be screened for diabetes mellitus at their first prenatal visit (12-14 weeks gestation), making B) 30 the correct answer. 1, 2

Screening Algorithm Based on BMI Categories

High-Risk Women (BMI ≥30 kg/m²)

• Screen immediately at first prenatal visit (typically 12-14 weeks) using either a 50g glucose challenge test or 75g oral glucose tolerance test 1, 2
• Mandatory repeat screening at 24-28 weeks if initial screening is negative, as insulin resistance increases exponentially in the second and third trimesters 1, 2
• This early screening detects pre-existing undiagnosed type 2 diabetes that was present before pregnancy, not just gestational diabetes that develops later 1, 2

Intermediate-Risk Women (BMI 25-29.9 kg/m²)

• Consider early screening at first prenatal visit only if additional risk factors are present, such as prior GDM history, family history of diabetes in first-degree relatives, or high-risk ethnicity 1, 2
• If no additional risk factors, follow standard screening at 24-28 weeks 2

Low-Risk Women (BMI <25 kg/m²)

• Standard screening at 24-28 weeks of gestation is appropriate 2
• Can skip screening entirely if all of the following criteria are met: age <25 years, normal body weight, no family history of diabetes, no history of abnormal glucose metabolism, no history of poor obstetric outcome, and not a member of high-risk ethnic group 3

Clinical Context for This Patient

Your patient (BMI = 34.5 kg/m²) is at significantly elevated risk and requires immediate screening. Her BMI calculation: 85 kg ÷ (1.57 m)² = 34.5 kg/m², which exceeds the threshold of 30 kg/m² 1, 2

• At 11 weeks gestation, she should undergo glucose testing as soon as feasible 1, 2
• Women with BMI ≥30 have a 4.14 times higher risk of GDM compared to normal-weight women 2, 4
• Obesity independently increases the risk of fetal macrosomia (up to 20% in untreated GDM) beyond diabetes effects alone 1

Critical Pitfalls to Avoid

Failing to rescreen at 24-28 weeks if early testing is negative is the most common error, leading to delayed diagnosis and increased maternal-fetal complications 1, 2

• Do not rely on fasting glucose alone for diagnosis—a full OGTT is required for definitive gestational diabetes diagnosis 1
• Do not delay screening in obese women; this patient should be tested immediately at her current visit 1
• Do not assume that absence of other risk factors (family history, prior GDM) negates the need for early screening when BMI ≥30 1, 2

Diagnostic Testing Methods

Two-Step Approach (Most Common in US)

• Initial 50g glucose challenge test (non-fasting) with threshold ≥130-140 mg/dL 2
• If positive, proceed to diagnostic 100g OGTT with thresholds: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL 3, 2

One-Step Approach (Alternative)

• Direct 75g OGTT with thresholds: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL 3, 1
• May be more cost-effective in high-risk populations like this patient 3, 1

Evidence Quality

The recommendation for BMI ≥30 as the screening threshold is supported by high-strength evidence from the American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians 1, 2. While the other BMI thresholds (25,35,40) may represent varying degrees of risk, only BMI ≥30 has consistent guideline support as the definitive threshold for mandatory early screening 1, 2.