EFW Percentile by Gestational Age (Hadlock)

Use your ultrasound’s estimated fetal weight (EFW) and gestational age to see the percentile and a typical range based on a Hadlock-style reference.

Gestational age (weeks + days)
Typical use: 18–40 weeks. We clamp to 14–42 if outside.
Estimated fetal weight (EFW) in grams
From your ultrasound report.
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Percentile is based on a Hadlock-style reference curve (approximate). Clinical interpretation depends on full context.

What this tool calculates

  • Percentile for an entered estimated fetal weight (EFW) at a given gestational age (GA).
  • A typical weight band at that GA for context.

Inputs and assumptions

  • GA is in weeks and days. Values are clamped to 14–42 weeks.
  • EFW is the ultrasound-reported estimate in grams, usually derived from Hadlock-style biometry formulas.
  • Population medians by week are taken from a smoothed, Hadlock-like reference curve; mid-gestation medians align with widely used tables.

Computation

  • Median at GA: linear interpolation between week medians.
  • Distribution model: lognormal approximation with a fixed log-SD (σ ≈ 0.25). This reflects the proportional spread seen in fetal weights.
  • Percentile: z = (ln(EFW) − ln(median)) / σ; percentile = normal CDF(z) × 100.
  • Typical band: approximate 10th–90th percentiles computed from the same lognormal model.

Scope and limits

  • Percentiles depend on the reference population and the EFW method used. Alternative standards (for example, INTERGROWTH-21st) may yield different centiles.
  • Clinical interpretation considers the full picture, including serial growth, amniotic fluid, Dopplers, and maternal factors. This tool is for education, not diagnosis.

Last updated: 6 Oct 2025

How to use this calculator

  1. Enter gestational age: weeks + days from your report (e.g., 28 w 3 d).
  2. Enter EFW (g): the estimated fetal weight in grams from the same scan.
  3. Calculate: view the percentile, the typical band, and the ~50th percentile for quick context.

How to read the result

Percentile compares your baby’s weight with peers at the same GA (e.g., 12th means heavier than 12% at that age).

Typical band (~10th–90th) shows a broad “normal” spread for counselling; values can be below/within/above this band.

~50th percentile (median) gives a quick “about average” reference at the selected GA.