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Fetal Weight Calculator

Step 1 / 3 Estimate Weight
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Enter three ultrasound measurements, head circumference (HC), abdominal circumference (AC) and femur length (FL), and the calculator returns an estimated fetal weight (EFW) in grams using the Hadlock HC/AC/FL equation. The estimate is for education and general awareness only, it is not a diagnosis: discuss any concern with your obstetric team.

How to estimate fetal weight

  1. 1

    Enter the measurements

    Use the values from the latest ultrasound report. Head circumference, abdominal circumference and femur length are all entered in centimetres. All three values are required for the estimate.

  2. 2

    Calculate

    The tool applies the Hadlock HC/AC/FL equation (Hadlock et al., 1985) and returns the estimated fetal weight (EFW) in grams.

  3. 3

    Review against reference values

    Compare the result with the reference weight percentiles for the gestational age in the table below. A typical 1 SD error band of about 8% applies.

  4. 4

    Discuss with your care team

    Share the measurements and the result with your midwife or obstetric team. A calculator can never replace clinical assessment.

The Hadlock HC/AC/FL equation

The tool uses the three-parameter Hadlock equation from 1985, combining head circumference, abdominal circumference and femur length:

log10(EFW) = 1.326 - 0.00326 * AC * FL
            + 0.0107 * HC + 0.0438 * AC
            + 0.158 * FL

EFW is in grams and every biometry input is in centimetres. This equation has a reported error of about 8% (1 SD) in term pregnancies.

Related Hadlock equations

Equation Inputs Notes
Hadlock-1 AC, FL Two-parameter, fastest estimate
Hadlock-2 BPD, AC, FL Uses the biparietal diameter
Hadlock-3 HC, AC, FL The equation this tool uses
Hadlock-4 BPD, HC, AC, FL Four-parameter variant
Shepard BPD, AC Early 1980s estimates only
INTERGROWTH-21 HC, AC International standard cohort

Accuracy and limits

  • Error bars are wide. At 1 SD, a 3,000 g estimate is really 3,000 g ± 240 g; for 95% confidence, double that.
  • Third-trimester macrosomia. Over 4,000 g, every formula tends to underestimate.
  • Extreme prematurity. Below 24 weeks, Hadlock loses accuracy; INTERGROWTH-21 performs better.
  • Anatomical anomalies. If the fetus has hydrocephalus, limb shortening or growth restriction, no single formula fits well.

Reference weight percentiles (Hadlock 1991)

The calculator returns a single estimate in grams; it does not compute a percentile. Use the table below to compare your result with population norms for the gestational age.

Weeks 10th 50th 90th
28 898 1168 1438
32 1641 1981 2321
36 2352 2804 3256
40 2985 3578 4169

This calculator is not a substitute for clinical judgment. Always discuss results with the obstetric team.

Frequently Asked Questions

The Hadlock equation used here has a standard deviation of roughly 8% for term pregnancies, meaning a 3,000 g estimate can be 2,760-3,240 g at 1 SD and 2,520-3,480 g at 2 SD. Accuracy falls further for preterm or macrosomic cases.

This tool uses the Hadlock HC/AC/FL equation (Hadlock et al., 1985), which combines head circumference, abdominal circumference and femur length. All three measurements are required: the estimate is not calculated when one of them is missing.

The calculator returns a single weight estimate, not a growth diagnosis. If the result is clearly below the reference values for the gestational age, raise it with your obstetric team, because only a clinician can assess growth restriction.

Enter all three measurements in centimetres. The result is given in grams. No gestational age is needed for the calculation, but you need it to compare the result with the reference table.

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