Hadlock Fetal Weight Calculator
Estimate fetal weight (EFW) from common ultrasound biometry using the Hadlock (BPD, HC, AC, FL) equation.
For educational use only. Clinical decisions should always rely on your obstetric care team and full ultrasound report.
What is a Hadlock calculator?
A Hadlock calculator is a tool used in obstetrics to estimate fetal weight from ultrasound measurements. It applies a validated statistical equation developed by Dr. Frank P. Hadlock and colleagues. In routine prenatal care, this estimate is often shown as EFW (Estimated Fetal Weight).
The purpose is not to produce an exact birth weight, but to provide a practical estimate that helps clinicians monitor growth trends over time. One isolated value is useful, but serial measurements are usually more informative than a single scan.
Measurements used in this calculator
BPD (Biparietal Diameter)
The transverse diameter of the fetal head measured on ultrasound.
HC (Head Circumference)
A circumference measurement of the fetal skull that contributes strongly to fetal size estimation.
AC (Abdominal Circumference)
Often the most growth-sensitive fetal measurement, especially in third-trimester growth assessment.
FL (Femur Length)
The length of the fetal femur, used as a long-bone indicator of fetal development.
Hadlock formula used on this page
This calculator uses the commonly cited 4-parameter Hadlock equation:
log10(EFW) = 1.3596 + 0.0064(HC) + 0.0424(AC) + 0.174(FL) + 0.00061(BPD×AC) − 0.00386(AC×FL)
where measurements are in centimeters and EFW is in grams. Since many ultrasound reports show values in millimeters, this tool automatically converts mm to cm before calculating.
How to use the Hadlock calculator correctly
- Enter BPD, HC, AC, and FL exactly as reported on ultrasound.
- Use millimeters (mm), which is the expected unit in this form.
- Optionally add gestational age to get simple context against an expected median trend.
- Compare with prior scans to evaluate growth pattern rather than relying on one number.
Interpreting the result
EFW is an estimate, not a diagnosis. Typical real-world error can be meaningful, especially near term. A difference of several hundred grams from actual birth weight is not uncommon.
- Lower-than-expected estimates may trigger further growth monitoring.
- Higher-than-expected estimates may prompt discussion of delivery planning and maternal factors.
- Trend over time is often more clinically useful than one isolated scan value.
Limitations and best practices
Even excellent formulas have limitations. Accuracy depends on image quality, operator technique, fetal position, and pregnancy-specific factors.
- Different Hadlock variants exist, each with slightly different input sets and performance profiles.
- Biometry can vary between sonographers and machines.
- Late-third-trimester estimates can have wider error margins.
- Clinical context (maternal history, Dopplers, fluid status, and growth trajectory) matters.
Frequently asked questions
Is this a medical diagnosis tool?
No. It is an educational estimator. Use it as a companion to professional medical care, not a replacement.
Can this predict birth weight exactly?
No. It gives a statistically derived estimate from fetal measurements, not a guaranteed final weight.
What if my value looks unusual?
Do not panic based on one calculation. Speak with your obstetric clinician, who can interpret the result in the context of your gestational age, scan quality, and overall pregnancy health.