Hadlock Four-Parameter Estimated Fetal Weight Calculator

Overview

Estimates fetal weight from biparietal diameter, head circumference, abdominal circumference and femur length using the Hadlock four-parameter equation.

Clinical Significance

Ultrasound estimated fetal weight has meaningful random and systematic error. Interpret against gestational-age standards, growth trend, measurement quality and the complete obstetric assessment.

When to Use

• Estimate fetal weight when all four ultrasound biometric measurements are available.
• Use measurements acquired using recognised obstetric ultrasound planes and technique.
• Assess the result against a locally adopted fetal-growth standard.

How It Is Calculated

• log10(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064HC + 0.00061(BPD×AC) + 0.0424AC + 0.174FL, with measurements in centimetres.
• The calculator validates the required inputs before applying the equation and rounds only the displayed result.

Interpretation

• Ultrasound estimated fetal weight has meaningful random and systematic error. Interpret against gestational-age standards, growth trend, measurement quality and the complete obstetric assessment.
• Review the component values and assumptions as well as the final result.

Worked Example

Enter clinically plausible values in the stated units. MedicalC applies: log10(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064HC + 0.00061(BPD×AC) + 0.0424AC + 0.174FL, with measurements in centimetres. Review the displayed result against the source data and the relevant clinical pathway.

Patient Considerations

• Confirm that the patient and clinical setting match the population or physiologic assumptions behind the equation.
• Consider whether treatment, organ function, pregnancy, body composition, sampling conditions or acute illness materially alter interpretation.
• Use serial calculations only when methods and units remain comparable.

Limitations

• Typical individual error can be clinically important, particularly at weight extremes.
• The equation is not a substitute for serial growth assessment.
• Incorrect units or biometric planes produce major errors.

Clinical Pearls

• All biometric inputs must be in centimetres.
• Review the percentile and growth trajectory, not the weight estimate alone.
• Recognise that management thresholds should account for the uncertainty of ultrasound EFW.

Common Mistakes

• Entering millimetres rather than centimetres.
• Using measurements from different examinations.
• Treating the estimate as an exact fetal weight.

Evidence Base

The implementation uses the published or established relationship: log10(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064HC + 0.00061(BPD×AC) + 0.0424AC + 0.174FL, with measurements in centimetres. The cited source should be reviewed alongside current specialty guidance before clinical deployment.

References

1. Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985. — Primary publication, authoritative guideline or recognised clinical source supporting the calculation.