BMI Threshold for Gestational Diabetes Screening
Pregnant women with BMI ≥30 kg/m² should be screened for gestational diabetes at their first prenatal visit (12-14 weeks), and if negative, must be rescreened at 24-28 weeks. 1, 2, 3
Answer: B - BMI 30
The correct answer is B (BMI 30), as this is the threshold at which major guideline organizations recommend early screening for gestational diabetes.
Screening Algorithm Based on BMI
BMI ≥30 kg/m² (Obese Women)
- Screen at first prenatal visit (12-14 weeks) using fasting plasma glucose or oral glucose tolerance test 1, 2, 3
- The purpose of early screening is to detect pre-existing undiagnosed type 2 diabetes, not just gestational diabetes 1, 3
- Mandatory repeat screening at 24-28 weeks if initial screen is negative, as insulin resistance increases exponentially in the second and third trimesters 2, 3
- This recommendation comes from the American College of Obstetricians and Gynecologists, American Diabetes Association, and American College of Physicians 1, 2
BMI 25-29.9 kg/m² (Overweight Women)
- Consider early screening at first prenatal visit only if additional risk factors are present (prior GDM, family history of diabetes, high-risk ethnicity, PCOS) 1, 2
- If early screening performed and negative, repeat at 24-28 weeks 1, 2
BMI <25 kg/m² (Normal Weight Women)
- Standard screening at 24-28 weeks unless other high-risk factors are present 1, 2
- May potentially skip screening if 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, and not from high-risk ethnic group 1
Universal Screening at 24-28 Weeks
Regardless of BMI, all pregnant women should undergo gestational diabetes screening between 24-28 weeks of gestation using either a one-step or two-step approach 4, 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 4, 1
- This is the standard screening window when gestational diabetes typically develops due to increasing insulin resistance 4, 5
Evidence Quality and Clinical Context
The BMI ≥30 threshold represents high-quality guideline consensus, though recent randomized controlled trial data suggests that early screening in obese women does not necessarily improve perinatal outcomes compared to routine screening 1, 6
- A 2020 RCT of 922 obese women found that early screening (14-20 weeks) versus routine screening (24-28 weeks) did not reduce composite perinatal outcomes (56.9% vs 50.8%, P=0.07) 6
- However, guideline organizations maintain the recommendation because early screening identifies pre-existing diabetes that requires immediate management 1, 3
Critical Pitfall to Avoid
Failing to rescreen at 24-28 weeks after negative early screening is a major error that can lead to delayed diagnosis and treatment of gestational diabetes that develops later in pregnancy 2, 3