Estimated fetal weight from your scan measurements
A growth scan report lists a few measurements — the head (BPD and HC), the tummy (AC) and the thigh bone (FL) — and an estimated fetal weight (EFW). This fetal weight calculator turns those measurements into a weight in grams, kilograms and pounds using the widely used Hadlock equations, and shows where the estimate sits on the World Health Organization fetal growth chart for the same week.
It is information only — not medical advice and not a diagnosis. Your scan report and your care team are the reference; use this page to understand the numbers, not to judge your baby’s growth.
How to use the Fetal Weight Calculator (EFW)
- Enter the weeks and days of pregnancy on the day of the scan.
- Choose mm or cm to match your report, then type the BPD, HC, AC and FL. AC and FL are needed; the others make the estimate more complete.
- Leave the formula on Automatic (Hadlock 3, the one the WHO charts use) or pick another.
- Read the estimated weight, the percentile on the WHO chart and the comparison of all formulas, then copy the result if you want to keep it.
Fetal Weight Calculator (EFW) features
Four Hadlock equations
AC + FL; BPD + AC + FL; HC + AC + FL; and BPD + HC + AC + FL — side by side.
WHO percentile
Your estimate placed against the WHO 2.5th–97.5th percentile curves for weeks 14 to 40.
Chart
The WHO 10th, 50th and 90th percentile curves with your estimate marked.
kg, grams and pounds
Plus millimetres or centimetres for the measurements.
How estimated fetal weight is calculated
Frank Hadlock and colleagues (American Journal of Obstetrics and Gynecology, 1985) measured babies shortly before birth and fitted equations linking the scan measurements to birth weight. Each equation gives log10 of the weight in grams from the measurements in centimetres. The one using HC, AC and FL — often called Hadlock 3 — is the formula the WHO used when it built its fetal growth charts, so it is the default here. The abdominal circumference has the biggest influence: a few millimetres there change the estimate more than anything else.
The WHO charts (Kiserud and colleagues, PLOS Medicine, 2017) followed 1,387 low-risk pregnancies in ten countries with repeated scans. This page uses their estimated-fetal-weight table for both sexes, interpolating between whole weeks and between percentile lines.
Reading the percentile calmly
- A percentile only compares one estimate with a reference population: at the 30th percentile, about 30 in 100 babies in the reference were lighter at that week.
- Estimates have a margin of error that grows later in pregnancy, and small differences in how a measurement is taken move the result.
- Hospitals use different charts (WHO, INTERGROWTH-21st, Hadlock’s own, customised charts), so the percentile on your report may differ from this one.
- Growth is judged over time, using several scans and other checks. Ask your midwife, sonographer or doctor to explain your results.
Related tools
See typical sizes for every week in the baby size by week chart, check your dates with the due date calculator, and from around 24–28 weeks get to know your baby’s movements with the kick counter.
Fetal Weight Calculator (EFW) FAQ
How is fetal weight calculated from an ultrasound?
The scan measures the head (BPD and HC), abdomen (AC) and femur (FL). An equation such as Hadlock’s turns them into an estimated weight; this calculator uses the four Hadlock 1985 equations.
How accurate is estimated fetal weight?
It is an estimate, not a weighing. Errors of several per cent are usual and larger differences happen, especially near term. Your care team takes this into account.
What is the fetal weight percentile?
Where the estimate sits compared with the WHO reference for the same week: at the 50th percentile half of babies were lighter and half heavier.
Which formula should I use?
Hadlock 3 (HC, AC, FL) is a common default and the one behind the WHO charts. If your report lists a different formula, choose it to compare like with like.
Can I calculate fetal weight in kg?
Yes. The result is shown in grams, kilograms and pounds and ounces.
My result is below the 10th or above the 90th percentile. What does it mean?
On its own, very little — many such babies are healthy. Only your care team can interpret it, using your full scan history. Ask them about your results.