Fetal Weight Estimate

Features

• Uses the Hadlock two-parameter formula (AC + FL) and prints the full coefficients for checking
• Reports a ±15% error band (wider for the smallest and largest babies, as the page explains)
• Echoes the two inputs used, so a mismatch with your report is easy to trace
• No “too small / too large” verdict — that is an obstetric judgement, not something a formula should assert
• Flags inputs outside the usual range and asks you to check the unit (cm mistaken for mm is the classic error)
• Gives the weight in kilograms and, in Chinese, in the traditional jin/liang without false precision
• Identical coefficients in all three languages: switching language never changes the number
• Computed locally; nothing is uploaded

How to use

1. Find the abdominal circumference (AC) and femur length (FL) on your scan report — usually in millimetres
2. Enter both; the result appears immediately
3. Read the estimate together with the error band below it — the range carries more information than the single figure
4. To cross-check, use the printed formula with the numbers on your own report

FAQ

Why is there no BPD (biparietal diameter)?

Because the coefficients here are the ones that could be checked. The AC + FL form was validated against typical measurements at 20, 24, 30 and 40 weeks, all landing inside the accepted weight range for those gestations. The BPD-based variants tried in the same pass (putting BPD where AC goes, or adding an AC×BPD interaction) produced obviously wrong magnitudes. Regression coefficients recalled from memory are not reliable — and on a tool that pregnant users will read, it is better to have one input fewer than a number nobody can reproduce.

My estimate differs from the one on my scan report. Which is right?

Both are estimates, and they are quite likely using different formulas. Hadlock exists in several versions (two, three and four parameters), different hospitals and different machine software use different ones, and different coefficients give different answers — a 10% spread is normal. That is why the full coefficients are printed here: run this formula with the measurements from your report. If they agree, the difference was the formula; if not, the inputs differed.

Where does the error band come from?

In the literature the standard deviation of Hadlock estimates against birth weight is roughly 7–8% of the mean, so ±2 standard deviations is around 15%, which is the band used here. Note that the error is larger at both extremes — very small and very large babies both carry more relative error. So this is a rough magnitude, not a confidence interval: fine for “roughly what range”, not for “is something wrong”.

Can I use this to tell whether the baby is too small?

No, and this tool deliberately gives no such verdict. Assessing growth restriction or macrosomia requires an accurate gestational age, the trend across several scans, amniotic fluid volume, umbilical artery flow, the mother’s history and glucose status — that is an obstetric diagnosis. A single isolated weight estimate can neither confirm nor exclude anything, so this page provides the numbers and leaves the reading to a professional.