At what Body Mass Index (BMI) should a pregnant individual undergo screening for diabetes mellitus?

BMI Threshold for Early Diabetes Screening in Pregnancy

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

Screening Algorithm Based on BMI

BMI ≥30 kg/m² (Answer: B)

• The American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians all recommend screening at the first prenatal visit (12-14 weeks) for women with BMI ≥30 kg/m² 1, 2, 3
• This early screening is designed to detect pre-existing undiagnosed type 2 diabetes that was present before pregnancy, not just gestational diabetes that develops later 1, 2
• If early screening is negative, mandatory repeat screening at 24-28 weeks is required, as insulin resistance increases exponentially in the second and third trimesters 1, 2, 3

BMI 25-29.9 kg/m² with Additional Risk Factors

• Women in this overweight category should consider early screening only if they have additional risk factors such as prior GDM, family history of diabetes, or high-risk ethnicity 2, 3
• Without additional risk factors, standard 24-28 week screening is appropriate 2

BMI <25 kg/m²

• Standard screening at 24-28 weeks unless other high-risk factors are present 1, 2
• Women meeting ALL low-risk criteria (age <25 years, BMI ≤25 kg/m², no abnormal glucose tolerance history, no adverse obstetric outcomes, no first-degree relative with diabetes, not from high-risk ethnic group) could potentially skip screening, though universal screening is most common practice 1

Evidence Supporting the BMI ≥30 Threshold

Risk Quantification

• Women with BMI >25 kg/m² have over 4 times the risk of developing GDM compared to normal-weight women 1
• The risk increases progressively: overweight women have OR 1.97, moderately obese women OR 3.01, and morbidly obese women OR 5.55 for developing GDM 4
• For every 1 kg/m² increase in BMI, the prevalence of GDM increases by 0.92% 4

Clinical Rationale

• The prevalence of undiagnosed type 2 diabetes has risen dramatically in women of reproductive age, particularly those with obesity 3
• Early pregnancy BMI is a significant independent risk factor for GDM (OR 4.14) 1
• Obesity independently increases the risk of fetal macrosomia beyond diabetes effects, with untreated gestational diabetes carrying up to 20% risk of macrosomia 3

Universal Screening at 24-28 Weeks

• All pregnant women, regardless of BMI, should undergo routine screening at 24-28 weeks of gestation 1, 2
• The U.S. Preventive Services Task Force gives a B recommendation (moderate net benefit) for universal screening after 24 weeks to reduce preeclampsia, macrosomia, and shoulder dystocia 5, 1
• There is insufficient evidence to support screening before 24 weeks in average-risk women 5, 1

Important Caveats

Recent Trial Data

• A 2024 randomized controlled trial found that early GDM screening in obese patients (BMI ≥30) does not improve perinatal outcomes compared to routine 24-28 week screening 6
• However, guideline recommendations from major societies still support early screening at BMI ≥30 to detect pre-existing diabetes, which requires more intensive management from the outset 1, 2, 3

Common Pitfalls to Avoid

• Do not skip the mandatory 24-28 week rescreen if early testing is negative in high-risk women - this leads to delayed diagnosis and increased maternal-fetal complications 2, 3
• Do not rely on fasting glucose alone for diagnosis - a full oral glucose tolerance test is required for definitive gestational diabetes diagnosis 3
• Do not delay screening in women already presenting at 16+ weeks with BMI ≥30 - test immediately 3