A routine third trimester growth ultrasound in the obese pregnant woman does not reliably identify fetal growth abnormalities: A retrospective cohort study

Background: In response to the challenges of assessing fetal growth in obese women, guidelines recommend routine third trimester ultrasound scans. Aim: The aim of this study was to assess the diagnostic performance of this routine scan in obese women (body mass index (BMI) >= 35 kg/m(2)). Methods: A retrospective cohort study of 1008 pregnancies with maternal BMI >= 35 kg/m(2) born after 37 weeks gestation at a Victorian hospital from 2015 to 2017. Multiple pregnancies and those affected by diabetes were excluded. Growth ultrasounds were performed between 34 + 0 and 36 + 6 weeks gestation. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for the detection of large for gestational age (LGA > 90%) and small for gestational age (SGA < 10%) were calculated using ultrasound estimated fetal weight (EFW) or abdominal circumference (AC) and compared with gestational age and gender-based birthweight percentiles. Results: Using EFW, sensitivity for detecting SGA at birth was 8.1% (six of 74) with a PPV of 100%. Sensitivity for detecting LGA at birth was 61.0% (119 of 195), PPV 54.8%. Sensitivity, specificity, PPV and NPV percentages were all lower using AC. Only 40% of actual birthweight percentiles (405/1008) were within +/- 10 percentiles of their growth ultrasound EFW percentile. Conclusion: The performance of a routine third trimester ultrasound in women with BMI >= 35 kg/m(2) suggests limited utility in helping identify aberrant fetal growth. This has important implications for the management of obese pregnant women.