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Obstetric Imaging
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Estimated Fetal Weight (EFW)

Estimate fetal weight from ultrasound biometry. Detect IUGR and macrosomia. Essential for delivery planning.

Estimated fetal weight (EFW) converts sonographic biometric measurements into a predicted weight in grams, providing a single summary metric of fetal growth. It is central to antenatal surveillance, allowing detection of fetal growth restriction (FGR/IUGR) and macrosomia, and it informs timing and mode of delivery. The Hadlock regression models remain the most widely used and validated worldwide.

🧮 Interactive Calculator

📐 Formula

Hadlock formula: log10(EFW) = 1.335 - 0.0034(AC)(FL) + 0.0316(BPD) + 0.0457(AC) + 0.1623(FL)

Reference: Hadlock 1985

When to Use

  • Routine third-trimester growth assessment, particularly when serial biometry is indicated
  • Suspected fetal growth restriction when fundal height lags or Doppler abnormalities are present
  • Suspected macrosomia in diabetic pregnancies or large-for-dates uterus, to inform mode of delivery
  • Pre-delivery planning for preterm birth, breech, or twin gestations where weight guides neonatal preparation
  • Assessing growth discordance in multiple pregnancies
  • Before administering antenatal interventions (e.g. steroids, MgSO4) where estimated size affects counselling

How It Works

  • The Hadlock 4-parameter model regresses log10(EFW) on biometric variables; the version shown uses AC, FL, BPD and an AC*FL interaction term.
  • Inputs are standard second/third-trimester ultrasound measurements: biparietal diameter (BPD), abdominal circumference (AC), femur length (FL), and in the full Hadlock model head circumference (HC).
  • Measurements are entered in centimetres; the equation yields log10 of the weight, so the antilog (10^result) gives EFW in grams.
  • The model assumes correctly obtained standard-plane measurements: BPD/HC at the thalami and cavum septi pellucidi, AC at the level of the stomach and portal sinus, FL as the full diaphyseal length.
  • EFW is then plotted on a gestational-age-specific growth chart to derive a centile, which is more clinically meaningful than the absolute weight.
  • Hadlock formulas were derived from a largely Caucasian US population; local or customised charts (e.g. INTERGROWTH-21st, GROW) may be substituted for centile assignment.

Interpreting the Result

  • EFW is conventionally reported with its gestational-age centile: below the 10th centile suggests small-for-gestational-age, above the 90th centile suggests large-for-gestational-age/macrosomia.
  • EFW or AC below the 3rd centile is a strong stand-alone criterion for fetal growth restriction per Delphi consensus.
  • FGR is more confidently diagnosed when a low EFW/AC centile is combined with abnormal umbilical or uterine artery Doppler.
  • EFW above ~4000-4500 g at term raises concern for shoulder dystocia and influences discussion of caesarean delivery, especially in diabetes.
  • Serial EFW showing a fall across centile lines (reduced growth velocity) is significant even if the absolute value remains above the 10th centile.
  • A single EFW is a snapshot; trend over 2-3 week intervals is more informative than any one measurement.

Limitations & Pitfalls

  • Random error of ±10-15% (roughly ±1 SD) means an individual estimate can be substantially off actual birth weight.
  • Accuracy degrades at the extremes: EFW tends to overestimate small fetuses and underestimate macrosomic ones.
  • Poor image quality, maternal obesity, oligohydramnios, fetal position, and advanced gestation reduce measurement precision.
  • Formulas assume normal body proportions; asymmetric growth restriction or skeletal dysplasia distorts estimates.
  • Population-specific bias exists because Hadlock reference data were not ethnically diverse; using an inappropriate growth chart misclassifies centiles.
  • Operator variability in obtaining standard planes is a major source of inter-observer disagreement.

References

  • Hadlock FP et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985.
  • Gordijn SJ et al. Consensus definition of fetal growth restriction: a Delphi procedure. Ultrasound Obstet Gynecol. 2016.
  • ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol. 2019.
  • Papageorghiou AT et al. International standards for fetal growth (INTERGROWTH-21st). Lancet. 2014.
  • ACOG Practice Bulletin No. 227: Fetal Growth Restriction. 2021.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.