Which Hadlock formula is most accurate?

The four-parameter Hadlock formula (BPD + HC + AC + FL) generally provides the best accuracy, with a reported mean error of about ±15%. When only AC is available, the AC-only formula is still clinically useful. Using more biometric parameters reduces random measurement error.

What units should I enter measurements in?

All biometry measurements (BPD, HC, AC, FL) should be entered in millimeters (mm), as reported on standard ultrasound reports. The gestational age is entered in completed weeks.

What do the EFW percentiles mean?

Percentiles compare the estimated fetal weight to a reference population at the same gestational age. Below the 10th percentile may indicate small for gestational age (SGA); above the 90th percentile may indicate large for gestational age (LGA). These are screening flags, not diagnoses.

How accurate is sonographic EFW?

Sonographic EFW has an inherent error of roughly ±10–20% regardless of the formula used. Accuracy decreases at extremes of weight and late gestational age. Always interpret EFW in the full clinical context.

What is AC in fetal biometry?

Abdominal circumference (AC) is the measurement around the fetal abdomen at the level of the stomach and umbilical vein, taken in the transverse plane. It is the single most sensitive biometric parameter for detecting fetal growth restriction.

Can I use this calculator for twin pregnancies?

The Hadlock formulas were derived from singleton pregnancies. They can be applied to twins as a rough guide, but twin-specific growth charts should be used for formal assessment. Consult your obstetrician for twin pregnancies.

Estimated Fetal Weight Calculator

Name: Estimated Fetal Weight Calculator
Availability: InStock
Author: Nham Vu

Enter ultrasound biometry measurements (BPD, HC, AC, FL) to estimate fetal weight using the Hadlock formula.

Ultrasound Biometry

All measurements in millimeters (mm). AC is required; others improve accuracy.

Gestational Age (weeks)

Used for percentile lookup (14–42 weeks)

Abdominal Circumference — AC *

Biparietal Diameter — BPD (optional)

Head Circumference — HC (optional)

Femur Length — FL (optional)

Enter AC and click Calculate EFW to see the result.

Summary

Enter ultrasound biometry measurements (BPD, HC, AC, FL) to estimate fetal weight using the Hadlock formula.

How it works

Enter gestational age and one or more biometry measurements (AC required).
The calculator selects the Hadlock formula variant matching your inputs.
Log10(EFW) is computed from the formula coefficients and converted to grams.
EFW in grams is converted to pounds and ounces for display.
The result is compared to Hadlock percentile tables by gestational age.

Use cases

Estimate fetal weight from routine obstetric ultrasound measurements.
Cross-check sonographer EFW calculations in clinical or educational settings.
Assess fetal growth against gestational age percentile benchmarks.
Compare single-parameter and multi-parameter Hadlock formula results.
Track fetal growth trends across serial ultrasound scans.
Education and training for obstetrics and radiology students.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu