How Accurate are Estimated Fetal Weight Ultrasound Measurements between 23 and 25 Weeks?

BACKGROUND AND PURPOSE:

  • Haragan et al (American Journal of Perinatology, 2018) evaluated the accuracy of estimated fetal weight (EFW) to predict birth weight (BW) among infants born during the periviable period
    • Authors also assessed the accuracy of EFW in SGA versus AGA neonates

METHODS:

  • Retrospective study
    • Women who delivered between 23w0d and 25w7d
    • EFW obtained within 7 days of delivery
  • EFW based on Hadlock-3 formula (HC, BPD, FL and AC)
  • Accuracy of EFW defined as
    • EFW − BW)/BW × 100%
  • Median absolute percentage difference between EFW and BW
    • |(EFW − BW)/BW × 100|
  • Excess error for EFW was defined as the absolute percentage difference between EFW and BW ≥20%

RESULTS:

  • 226 neonates were involved in the study
    • Mean gestational age at birth: 24w1d
    • 11% SGA; 10% BW <500g
    • 74% spontaneous preterm
    • Median BW 653 g
  • Median absolute percentage difference between EFW and BW of fetal weight estimates was 9.2%
  • EFW overestimated BW in 75.7% of the cohort
  • 19% of cohort had excess error of ≥20%
    • More likely in scans that overestimated vs underestimated EFW (22.2% vs 7.3%)
  • Among SGA infants
    • EFW was a poorer predictor of BW among SGA infants
    • Median absolute difference was greater among SGA infants (12.3%) vs AGA (8.6%), but did not achieve significance; p = 0.062
    • Mean percentage difference was 16.2 ± 19.4% in SGA versus 6.9% ± 13.1% in AGA infants; p = 0.019
  • Infants <500 g
    • Median absolute difference was significantly higher in infants <500g (15.2%) vs >500 g (8.6); p = 0.021
  • Mean percentage difference was greater among infants <500 g (18.2 ± 19.3%) versus 6.7 ± 13.0%; p = 0.003

CONCLUSION:

  • EFW is a “reasonable proxy” for BW in periviable fetuses
  • There are important limitations, especially in SGA or <500g
    • Ultrasound is likely to underestimate the true BW in these scenarios
  • Authors suggest that alternate ultrasound growth formulae should perhaps be developed, particularly infants predicted to be less than <500g

Learn More – Primary Sources:

Accuracy of Ultrasound Estimated Fetal Weight in Small for Gestational Age and Appropriate for Gestational Age Grown Periviable Neonates