Hadlock Estimated Fetal Weight

Estimates fetal weight from four standard ultrasound biometric measurements (BPD, head circumference, abdominal circumference, and femur length) using the Hadlock four-parameter formula (1985). Output is the estimated fetal weight in grams, with the characteristic ±1 SD error of approximately 7.5%.

Obstetrics & Gynecology ObstetricsEvidence-BasedUpdated 2026-08-16

Clinical Notes

Formula

log₁₀(EFW) = 1.3596 − 0.00386(AC × FL) + 0.0064(HC) + 0.00061(BPD × AC) + 0.0424(AC) + 0.174(FL); EFW in grams

Normal Range

Fetal weight depends on gestational age; the formula error is ±1 SD ≈ 7.5% (about ±8% in the original report).

Clinical Notes

The Hadlock four-parameter model (1985, Am J Obstet Gynecol 151:333-337) estimates fetal weight from biparietal diameter, head circumference, abdominal circumference, and femur length: log₁₀ EFW = 1.3596 − 0.00386(AC × FL) + 0.0064(HC) + 0.00061(BPD × AC) + 0.0424(AC) + 0.174(FL). The reported error is ±1 SD ≈ 7.5%. This calculator implements the full four-parameter model; all four inputs are required.

References

Hadlock FP, et al. Am J Obstet Gynecol. 1985;151(3):333-337.

Calculator Information

Specialty

Obstetrics

📋 Reference Ranges

Normal Range

Fetal weight depends on gestational age; the formula error is ±1 SD ≈ 7.5% (about ±8% in the original report).

Reference ranges for this calculator
Classification	Range
Estimated fetal weight Hadlock 4-parameter model (BPD, HC, AC, FL)	grams

What This Calculator Measures

Estimates fetal weight from the Hadlock four-parameter model (1985) using biparietal diameter, head circumference, abdominal circumference, and femur length, output as estimated fetal weight in grams.

How to Use

Obtain BPD, HC, AC, and FL from a standard fetal ultrasound biometric assessment.
Enter each measurement in centimeters.
Review the estimated fetal weight and its ± 1 SD (≈ 7.5%) error band.

Interpretation Guide

The formula returns an estimated fetal weight with an error of approximately ± 7.5% (1 SD). Compare the result with gestational-age growth charts; an EFW below the 10th percentile suggests growth restriction, and above the 90th percentile suggests macrosomia, but serial measurements are far more reliable than a single value.

Age-specific interpretation

Clinical Significance

The Hadlock four-parameter formula is a widely used, externally validated method for estimating fetal weight from routine ultrasound biometry, supporting the diagnosis of growth restriction and macrosomia and informing delivery planning.

When to Use

Suspected fetal growth restriction or macrosomia
Gestational age dating when ultrasound biometry is performed
Serial growth assessment in high-risk pregnancies

When Not to Use

As an exact birth-weight predictor (all formulas carry substantial error)
As a substitute for antenatal fetal surveillance when fetal status is in question

Limitations

1 SD error ≈ 7.5% (± ~250 g near term) — treat the value as an estimate, not a measurement.
Accuracy declines at the extremes of fetal size and in late gestation.
Operator-dependent measurements propagate into the EFW.

Worked Example

A 34-week ultrasound shows BPD 9.4 cm, HC 33.0 cm, AC 32.5 cm, and FL 7.0 cm.

Inputs:

bpd: 9.4

hc: 33.0

ac: 32.5

fl: 7.0

Estimated fetal weight 2,985 g (≈ 6.6 lb), ± 1 SD ≈ ± 224 g (7.5%).

Evidence

Source: Obstetrics / Ultrasound
Reference: Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements — a prospective study. Am J Obstet Gynecol. 1985;151(3):333-337.
Reviewed By: MedCalcHub Clinical Team
Version: 1.0
Updated: 2026-08

Hadlock FP, et al. Am J Obstet Gynecol. 1985;151(3):333-337.
ACOG Practice Bulletin No. 227: Fetal growth restriction. Obstet Gynecol. 2021;137(2):e16-e28.

References

Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements — a prospective study. Am J Obstet Gynecol. 1985;151(3):333-337. (Original Description)
ACOG Practice Bulletin No. 227: Fetal growth restriction. Obstet Gynecol. 2021;137(2):e16-e28. (Guideline)

Frequently Asked Questions

How accurate is the Hadlock formula?

The 1 SD error is approximately 7.5% (roughly ±250 g near term). This means a computed EFW of 3000 g should be read as approximately 2775–3225 g.

Why are all four measurements required?

This calculator uses the four-parameter Hadlock model (BPD, HC, AC, FL), which has better accuracy than models using fewer measurements. Three-parameter and two-parameter variants exist but are less precise.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Estimated fetal weight is inherently imprecise and must be interpreted with gestational age, growth trends, and clinical judgment.