BMI Threshold for Diabetes Screening in Pregnant Women with Family History of Type 2 Diabetes
A pregnant woman with a family history of type 2 diabetes should be screened for diabetes at BMI ≥30 kg/m² at her first prenatal visit. The correct answer is B. 30.
Screening Recommendations Based on BMI and Risk Factors
Women with BMI ≥30 kg/m² should undergo early diabetes screening at their first prenatal visit (12-14 weeks of gestation), regardless of other risk factors. 1, 2, 3 This early screening is designed to detect pre-existing undiagnosed type 2 diabetes, not just gestational diabetes mellitus (GDM), which typically develops later in pregnancy. 1, 2
- The American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians all recommend this BMI threshold of ≥30 kg/m² for early screening. 1, 2, 3
- Family history of diabetes in first-degree relatives is an independent risk factor (OR 1.76) that strengthens the indication for early screening when combined with elevated BMI. 1
- Women with BMI ≥30 kg/m² have over 4 times the risk of developing GDM compared to normal-weight women. 2
Complete Screening Algorithm for This Patient
If early screening at the first prenatal visit is negative, mandatory repeat screening at 24-28 weeks of gestation is required. 1, 2, 3 This is critical because insulin resistance increases exponentially in the second and third trimesters, and GDM typically develops during this period. 3
Early Screening (First Prenatal Visit):
- Perform fasting plasma glucose testing; a value ≥126 mg/dL indicates overt diabetes requiring immediate treatment. 3
- Alternatively, use a 75-g oral glucose tolerance test (OGTT) or 50-g glucose challenge test. 3
Standard Screening (24-28 Weeks):
- All pregnant women, including those with negative early screening, should undergo routine GDM screening at 24-28 weeks. 4, 1
- Use either a two-step approach (50-g glucose challenge test followed by 100-g OGTT if abnormal) or one-step approach (75-g OGTT). 4, 1
Why Other BMI Thresholds Are Incorrect
BMI 25 kg/m² (Option A) is not the established threshold for mandatory early screening. Women with BMI 25-29.9 kg/m² with additional risk factors may be considered for early screening, but this is not the standard recommendation. 1 The 2008 ADA guidelines mentioned that women with BMI ≤25 kg/m², age <25 years, and no other risk factors could potentially skip screening entirely, but this was an exclusion criterion, not a screening threshold. 4, 2
BMI 35 kg/m² (Option C) and BMI 40 kg/m² (Option D) are too high. These thresholds would miss a significant proportion of high-risk women who should receive early screening. The evidence-based threshold is BMI ≥30 kg/m². 1, 2, 3
Critical Pitfalls to Avoid
- Failing to rescreen at 24-28 weeks if early screening is negative can lead to missed diagnoses of GDM, as the disease typically develops in the second or third trimester. 3
- Using HbA1c for GDM screening at 24-28 weeks is not recommended, as it does not perform as well as glucose tolerance testing for GDM diagnosis. 3
- Not performing early screening in women with BMI ≥30 kg/m² may result in delayed intervention and increased risk of maternal and fetal complications. 1
Postpartum Follow-up
Women diagnosed with GDM should be screened for persistent diabetes at 4-12 weeks postpartum using a 75-g OGTT with non-pregnancy diagnostic criteria. 4, 3 Lifelong screening for diabetes or prediabetes should continue at least every 3 years thereafter, as women with a history of GDM have a 10-fold increased risk of developing type 2 diabetes and an absolute risk that increases linearly over time (20% at 10 years, 50% at 40 years). 4