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

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Updated Outcomes Data for Neonates less than 500 Grams at Birth

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When LMP and Ultrasound Dates Don’t Match: When to Redate?