At what Body Mass Index (BMI) is screening for diabetes mellitus recommended for pregnant individuals?

BMI Threshold for Early Diabetes Screening in Pregnancy

Pregnant women with a BMI ≥30 kg/m² should be screened for diabetes mellitus at their first prenatal visit (12-14 weeks of gestation), making option B (30) the correct answer. 1, 2, 3

Screening Algorithm by BMI Category

BMI ≥30 kg/m² (Obese)

• Screen at first prenatal visit (12-14 weeks) using standard diagnostic criteria to detect pre-existing undiagnosed type 2 diabetes 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 2, 3
• This BMI threshold represents the strongest evidence-based cutoff endorsed by the American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians 1, 2

BMI 25-29.9 kg/m² (Overweight)

• Consider early screening at first prenatal visit only if additional risk factors are present (prior GDM, family history of diabetes, high-risk ethnicity, history of macrosomia, PCOS) 1, 3
• If no additional risk factors, follow standard screening at 24-28 weeks 1

BMI <25 kg/m²

• Standard screening at 24-28 weeks is sufficient 1
• May skip screening entirely if age <25 years, normal body weight, and no family history of diabetes 1

Rationale for BMI ≥30 kg/m² Threshold

The BMI ≥30 kg/m² cutoff is specifically chosen because:

• Significantly elevated risk: Women with BMI ≥30 have a 4.14 times higher risk of gestational diabetes compared to women with normal BMI 4
• Detection of pre-existing diabetes: Early screening at 12-14 weeks identifies undiagnosed type 2 diabetes that was present before pregnancy, not just gestational diabetes that develops later 2
• Rising prevalence: The prevalence of undiagnosed type 2 diabetes has increased dramatically in women of reproductive age, particularly those with obesity 2
• Macrosomia risk: Obesity independently increases the risk of fetal macrosomia beyond diabetes effects, with untreated gestational diabetes carrying up to 20% risk of macrosomia 2

Testing Methods for Early Screening

When screening at first prenatal visit:

• Two-step approach: Start with 50g glucose challenge test (GCT) in non-fasting state; if ≥130-140 mg/dL, proceed to diagnostic 100g OGTT 2, 3
• One-step approach: Direct 75g OGTT may be more cost-effective in high-risk populations like women with BMI ≥30 1, 2
• Diagnostic thresholds for overt diabetes: Fasting plasma glucose ≥126 mg/dL or random plasma glucose ≥200 mg/dL with symptoms 1

Critical Pitfalls to Avoid

• Failing to rescreen at 24-28 weeks: Not repeating screening in high-risk women who initially test negative leads to delayed diagnosis and increased maternal-fetal complications, as gestational diabetes typically develops later in pregnancy 2, 3
• Using fasting glucose alone: Fasting blood glucose is insufficient for gestational diabetes diagnosis; a full OGTT is required for definitive diagnosis 2
• Delaying screening: Women with BMI ≥30 should be tested immediately at first prenatal visit, not delayed until 24-28 weeks 2

Why Other BMI Thresholds Are Incorrect

• BMI 25 (Option A): This threshold alone does not warrant early screening unless additional risk factors are present 1, 3
• BMI 35 and 40 (Options C and D): These are unnecessarily high thresholds that would miss many high-risk women; the evidence-based cutoff is BMI ≥30 1, 2, 3