Prospective population-based cohort study of maternal obesity as a source of error in gestational age estimation at 11–14 weeks

Introduction: An impact of maternal obesity on ultrasound dating of pregnancy at 11–14 gestational weeks is possible and was investigated. Material and methods: A prospective cohort study based on the Danish national population during a 4-year period in which we entered all mothers with singleton pregnancies who had a known last menstrual period (LMP), a recorded booking of body mass index (BMI), and a late first trimester ultrasound dating scan using crown-rump-length measurement (gestational age 11⁺⁰–13⁺⁶ weeks). Almost all scans were performed transabdominally. Transvaginal ultrasound was only performed in the case of limited visibility by transabdominal scanning. Differences between LMP and ultrasound estimated date of delivery (EDD) were stratified by BMI classes. Odds ratios (ORs) were calculated and adjusted for maternal age, parity and smoking. Results: In total, 187 486 women were analyzed: 21.8% were overweight and 12.3% obese. Ultrasound EDD was ≥7 days later than by LMP in 5.8% of normal-weight women, 7.3% of obese women, and 10.0% of women with morbid obesity. Compared with normal BMI (18.5–24.9), the OR for postponing EDD increased with increasing BMI; BMI 25–29.9 [OR 0.97, 95% confidence interval (CI) 0.93–1.02], BMI 30–34.9 (OR 1.14, 95% CI 1.07–1.23), BMI 35–39.9 (OR 1.28, 95% CI 1.15–1.42), and BMI 40+ (OR 1.73, 95% CI 1.50–1.98). Lean pregnant women (BMI <18.5) also had a higher chance of having EDD postponed 7 days or more (OR 1.11, 95% CI 1.01–1.22). Conclusion: Rising maternal BMI appears to be associated with postponement of ultrasound EDD.