Identifying fetal growth disorders using ultrasound in obese nulliparous women

Objective: We evaluated the ability of third trimester ultrasound to diagnose fetal growth disorders among obese women. Methods: This is a retrospective cohort study of obese nulliparous women who delivered term singleton neonates who had an ultrasound within 5 weeks of delivery. We characterized the sensitivity, specificity, positive predictive value and negative predictive value of ultrasound to detect large-for-gestational age (LGA) and small-for-gestational age (SGA) infants. Antenatally, LGA and SGA were defined as an ultrasound estimated fetal weight >90% or <10% based on the Hadlock formula. Postnatally, LGA or SGA designation was based on gestational age-based birthweight percentiles. Test characteristics were analyzed for the total cohort and by class of obesity (class II, body mass index [BMI] = 35.0–39.9 kg/m² versus class III, BMI ≥ 40 kg/m²). We compared the area under the curve for receiver-operating characteristic (ROC) curves for different classes of obesity. Results: Of 690 women, 13 (1.9%) screened positive for SGA and 19 (2.8%) delivered an SGA neonate. In contrast, 158 (22.9%) screened positive for LGA and 97 (14.1%) delivered an LGA neonate. The sensitivity of ultrasound for SGA was 26.3% and the specificity was 98.8%. The sensitivity for LGA was 75.3% and the specificity was 85.7%. The ROC curves did not differ significantly for different classes of obesity (p =.69 for SGA, p =.75 for LGA). Conclusion: Ultrasound in obese women who delivered term pregnancies has a high specificity but poor sensitivity for SGA and a low positive predictive value for LGA.