Calculator inputs
Default input units are millimeters because that is how most ultrasound biometry is recorded. The page keeps a single-column content flow, while the form itself shifts to 3 columns on large screens, 2 on medium screens, and 1 on mobile.
Example data table
These rows are illustrative and are calculated in the page with the same Hadlock 4 equation used by the tool.
| Gestational Age | BPD (mm) | HC (mm) | AC (mm) | FL (mm) | Formula | Estimated Weight (g) |
|---|---|---|---|---|---|---|
| 28w 0d | 72.0 | 260.0 | 245.0 | 52.0 | Hadlock 4 | 1,217.4 |
| 32w 0d | 82.0 | 300.0 | 285.0 | 62.0 | Hadlock 4 | 1,992.0 |
| 36w 0d | 91.0 | 330.0 | 325.0 | 71.0 | Hadlock 4 | 2,977.9 |
Formula used
The calculator converts BPD, HC, AC, and FL from millimeters to centimeters before calculation. It then evaluates whichever formulas have enough measurements available.
Shepard
log10(EFW) = -1.7492 + 0.166×BPD + 0.046×AC − 2.646×(AC×BPD)/1000
Hadlock 1
log10(EFW) = 1.304 + 0.05281×AC + 0.1938×FL − 0.004×AC×FL
Hadlock 2
log10(EFW) = 1.335 − 0.0034×AC×FL + 0.0316×BPD + 0.0457×AC + 0.1623×FL
Hadlock 3
log10(EFW) = 1.326 − 0.00326×AC×FL + 0.0107×HC + 0.0438×AC + 0.158×FL
Hadlock 4
log10(EFW) = 1.3596 − 0.00386×AC×FL + 0.0064×HC + 0.00061×BPD×AC + 0.0424×AC + 0.174×FL
Optional growth-reference comparison
ln(weight in g) = 0.578 + 0.332×MA − 0.00354×MA²
MA means menstrual age in weeks. The growth-reference comparison is optional and is meant for context, not percentile assignment.
How to use this calculator
- Enter any ultrasound biometry you have available. More measurements unlock more formulas.
- Choose your preferred formula. If it lacks enough inputs, the page will use the first available equation and tell you.
- Optionally add gestational age for a reference-mean comparison and chart marker.
- Adjust the uncertainty band if you want a broader counseling or documentation range.
- Press the calculate button. The result appears above the form and under the header.
- Review the comparison table to see how different formulas behave with the same measurements.
- Use the CSV and PDF buttons to export the calculation summary, formula table, and graph.
FAQs
1) What does estimated fetal weight mean?
Estimated fetal weight is an ultrasound-based approximation of fetal size. It is calculated from biometric measurements, not measured directly, so the value should be interpreted as a clinical estimate.
2) Which formula should I choose first?
Hadlock 4 is often preferred when all four measurements are available because it uses the richest biometric set. This page also shows other equations so you can compare agreement.
3) Why do formulas give different answers?
Each equation was derived from a different regression model and variable combination. Head shape, fetal position, image quality, and abdominal size can shift one formula higher or lower.
4) Are the ranges exact confidence intervals?
No. The range shown here is an illustrative band chosen from the percentage you enter. It is helpful for review and counseling, but it is not a formal statistical confidence interval.
5) Does this calculator assign percentiles?
No. It compares the estimate with an optional reference mean when gestational age is provided. True percentile assignment should use an appropriate chart for the pregnancy context.
6) Can I use only two measurements?
Yes. Shepard needs BPD and AC, while Hadlock 1 needs AC and FL. If you enter more measurements, additional formulas become available automatically.
7) Why is gestational age optional?
Weight estimation can be calculated from biometrics alone. Gestational age is only needed when you want the extra growth-reference comparison and the gestational-age chart marker.
8) Can this replace a clinician's interpretation?
No. Ultrasound estimation should be interpreted with the full clinical picture, including dating, interval growth, amniotic fluid, Dopplers, and the reporting clinician’s judgment.