BMI Threshold for Gestational Diabetes Screening
The correct answer is B) 30 kg/m². Pregnant women with a BMI ≥30 kg/m² should be screened for diabetes at their first prenatal visit (12-14 weeks) and again at 24-28 weeks if initially negative. 1, 2, 3
Screening Algorithm Based on BMI
High-Risk Women (BMI ≥30 kg/m²)
- Screen at first prenatal visit (12-14 weeks) using fasting plasma glucose or oral glucose tolerance test to detect pre-existing undiagnosed type 2 diabetes 1, 2, 3
- Mandatory repeat screening at 24-28 weeks if initial screening is negative, as insulin resistance increases exponentially in the second and third trimesters 1, 3
- This recommendation comes from the American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians 1, 2
Overweight Women (BMI 25-29.9 kg/m²)
- Consider early screening at first prenatal visit only if additional risk factors are present (prior GDM, family history of diabetes, high-risk ethnicity) 1, 3
- Repeat screening at 24-28 weeks if initial screen is negative 1
Normal Weight Women (BMI <25 kg/m²)
- Standard screening at 24-28 weeks unless other high-risk factors are present 1, 2
- May skip screening entirely if meeting ALL of the following: age <25 years, normal body weight, no family history of diabetes, no adverse obstetric outcomes, and not from high-risk ethnic group 2
Clinical Context for Your Patient
For this specific case: The patient's BMI is 34.5 kg/m² (85 kg ÷ 1.57² m), which exceeds the threshold of 30 kg/m². She should have been screened at her first prenatal visit and requires immediate screening now, followed by repeat screening at 24-28 weeks if negative. 1, 3
Evidence Supporting BMI ≥30 Threshold
The BMI ≥30 threshold is supported by strong evidence:
- Women with BMI >25 kg/m² have over 4 times the risk of developing GDM compared to normal-weight women (OR 4.14) 2, 4
- BMI ≥35 kg/m² carries even higher risk (OR 6.1) for GDM 5
- Early screening at BMI ≥30 is intended to identify pre-existing type 2 diabetes that was undiagnosed before pregnancy, not just gestational diabetes that develops later 1, 3
Why Not the Other Options?
- BMI 25 (Option A): While overweight, this threshold alone does not warrant early screening unless additional risk factors are present 1, 3
- BMI 35 and 40 (Options C & D): These are unnecessarily high thresholds that would miss many high-risk women who should be screened earlier 1, 2
Common Pitfalls to Avoid
- Failing to rescreen at 24-28 weeks: Even if early screening is negative in high-risk women, mandatory repeat screening is required as gestational diabetes typically develops later in pregnancy 1, 3
- Using fasting glucose alone: A full oral glucose tolerance test is required for definitive diagnosis; fasting glucose alone is insufficient and may miss postprandial hyperglycemia 3
- Delaying screening: This patient at 16 weeks with BMI >30 should be tested immediately 3