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

Aekta Neel, Chris E Cunningham, Glyn R Teale
Australian & New Zealand Journal of Obstetrics & Gynaecology 2020 October 24

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/m2 ).

METHODS: A retrospective cohort study of 1008 pregnancies with maternal BMI ≥ 35 kg/m2 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/m2 suggests limited utility in helping identify aberrant fetal growth. This has important implications for the management of obese pregnant women.

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