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Fetal Growth Calculator

Estimate fetal weight from three ultrasound measurements (BPD, AC and FL) with the Hadlock 1985 formula and compare it with the Hadlock average weight for the week of pregnancy.

Last updated: October 2026

Estimated fetal weight

2005 g (4 lb 7 oz) estimated fetal weight; 3% above the Hadlock average of 1953 g for 32 weeks

This calculator gives an estimate for general information only. It is not medical advice and does not replace a doctor, pharmacist or other qualified health professional. Always check with them before acting on the result, especially for medication doses, children, pregnancy or a medical condition, and follow the guidelines of your local health authority.

Formula below · 3 sources (pubmed.ncbi.nlm.nih.gov, findacode.com, acog.org) · Updated Oct 2026

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About this calculator

Ultrasound cannot weigh a baby directly, so the weight is estimated from body measurements. This calculator uses the Hadlock 1985 formula that combines three of them, with every length in centimeters:

log10(weight in g) = 1.335 − 0.0034 × AC × FL + 0.0316 × BPD + 0.0457 × AC + 0.1623 × FL

BPD is the biparietal diameter (the width of the head), AC the abdominal circumference and FL the femur length. You enter them in millimeters, as they appear on most scan reports, and the calculator converts them.

For comparison, the result also shows the average weight for the week from Hadlock's 1991 fetal weight standard, ln(weight) = 0.578 + 0.332 × week − 0.00354 × week², and how far above or below it the estimate is. The gestational age does not change the estimate itself.

How accurate is it?

Even with good images, ultrasound weight estimates are commonly off by 10% or more from the weight at birth, and the error grows late in pregnancy. Clinicians look at the trend over several scans rather than one number. An estimated weight below the 10th percentile for the week is the threshold ACOG uses for fetal growth restriction, but many of those babies are simply small and healthy.

This is an estimate, not medical advice. Your sonographer and clinician interpret your scans.

How to use

Copy BPD, AC and FL in millimeters from the scan report and enter the gestational age on the day of the scan. Example (the defaults): at 32 weeks with BPD 82 mm, AC 285 mm and FL 62 mm, the formula uses 8.2, 28.5 and 6.2 cm: 1.335 − 0.601 + 0.259 + 1.302 + 1.006 = 3.302, and 10^3.302 ≈ 2005 g (4 lb 7 oz). The Hadlock average at 32 weeks is 1953 g, so the estimate is 3% above average. At 36 weeks with BPD 89 mm, AC 310 mm and FL 69 mm, the estimate is 2664 g, 5% below the 36-week average of 2813 g.

Frequently asked questions

What do BPD, AC and FL mean on my scan report?

BPD (biparietal diameter) is the width of the baby's head from one side to the other. AC (abdominal circumference) is the distance around the belly at the level of the liver. FL (femur length) is the length of the thigh bone. The report may also list HC (head circumference); the Hadlock formula used here does not use it.

How accurate is an ultrasound weight estimate?

Typically within about 10 to 15% of the birth weight, and less precise near term. For a 2,000 g estimate that means the real weight could be a few hundred grams either way. That is why growth is judged over several scans.

My estimate is below average. Should I worry?

A few percent above or below average is normal. ACOG defines fetal growth restriction as an estimated weight or abdominal circumference below the 10th percentile for the week, which is roughly 16% below the Hadlock average. Your clinician will look at the growth trend, fluid levels and blood-flow tests before drawing conclusions.

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Sources & references

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