Why isn’t this the “Hadlock formula” I see in clinic?

The Hadlock estimated-fetal-weight formula combines BPD, HC, AC, and FL measurements taken during an ultrasound. This tool does not have those numbers; it returns the population 50th-percentile fetal weight by gestational age from the same author’s reference table.

My ultrasound EFW is below the 10th percentile — what does that mean?

It can indicate fetal growth restriction (FGR), but interpretation depends on serial measurements, fluid level, Doppler studies, and other factors. Discuss with your obstetric care team — this tool does not substitute for that workup.

How accurate is the Hadlock 1991 reference?

It’s widely used in the U.S. and remains a common reference standard. Other populations use INTERGROWTH-21st or WHO fetal growth charts; reference choice can shift percentile assignment slightly.

What gestational ages does it cover?

Hadlock 1991 publishes percentile fetal weights from 10 to 40 weeks of gestational age. Below week 10, fetal weight estimates are clinically negligible.

Is my data stored anywhere?

No. Everything runs in your browser.

Fetal Weight Estimator

Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this fetal weight estimator

Enter the completed gestational age in weeks (10–40).
Pick the display units — grams, or pounds + ounces.
Press “Look up” to see the 10th, 50th, and 90th-percentile reference fetal weights for that week.
Use Copy to save the result or Reset to clear the form.

About this fetal weight estimator

The fetal weight estimator looks up the 10th, 50th, and 90th-percentile reference fetal weights by gestational-age week from the Hadlock fetal growth reference (Hadlock FP, Harrist RB, Martinez-Poyer J. “In utero analysis of fetal growth: a sonographic weight standard.” Radiology 1991;181:129-33). This is a population reference: the 50th percentile is the median expected weight for a fetus at that gestational age, not a prediction of your baby’s actual weight. The 10th and 90th columns show the population spread.

Critically, this tool does NOT use the Hadlock ultrasound estimated-fetal-weight formula — that formula needs biometric measurements (BPD, HC, AC, FL) taken by a sonographer and is a clinical instrument. This is a much simpler “what is the typical fetal weight at GA week X” lookup, useful for context when reading clinical notes or anticipating birth weight, but it cannot tell you whether a specific fetus is growth-restricted or large for gestational age.

Worked example: at week 32 the Hadlock 1991 reference gives a 50th-percentile fetal weight of 1953 g (about 4 lb 5 oz), with a 10th-percentile boundary at 1465 g and a 90th-percentile boundary at 2441 g.

This is a population reference and is not a substitute for an ultrasound biometry scan or for advice from a qualified clinician. This calculator is for general educational purposes only and is not medical advice — consult a licensed healthcare professional for personal guidance.

FAQ

Why isn’t this the “Hadlock formula” I see in clinic?: The Hadlock estimated-fetal-weight formula combines BPD, HC, AC, and FL measurements taken during an ultrasound. This tool does not have those numbers; it returns the population 50th-percentile fetal weight by gestational age from the same author’s reference table.
My ultrasound EFW is below the 10th percentile — what does that mean?: It can indicate fetal growth restriction (FGR), but interpretation depends on serial measurements, fluid level, Doppler studies, and other factors. Discuss with your obstetric care team — this tool does not substitute for that workup.
How accurate is the Hadlock 1991 reference?: It’s widely used in the U.S. and remains a common reference standard. Other populations use INTERGROWTH-21st or WHO fetal growth charts; reference choice can shift percentile assignment slightly.
What gestational ages does it cover?: Hadlock 1991 publishes percentile fetal weights from 10 to 40 weeks of gestational age. Below week 10, fetal weight estimates are clinically negligible.
Is my data stored anywhere?: No. Everything runs in your browser.