Estimated Fetal Weight (Hadlock) Calculator
Estimate fetal weight from ultrasound biometry — biparietal diameter, head and abdominal circumference and femur length — with the Hadlock formula and a rough percentile band.
How it works
log₁₀(EFW) = 1.3596 + 0.0064·HC + 0.0424·AC + 0.174·FL + 0.00061·BPD·AC − 0.00386·AC·FL (Hadlock 1985, cm → grams)
Sonographers estimate weight by plugging four standard measurements into regression formulas fitted to thousands of babies. Hadlock’s four-parameter equation is the most widely used; all such formulas carry a random error of about 10–15%, larger for very big or very small babies. The percentile band compares the estimate with the in-utero growth curve for the week, which is how “small” or “large for gestational age” is flagged — a trend across two scans a fortnight apart matters more than one reading.
Worked example
BPD 78, HC 285, AC 265, FL 58 mm at 31 weeks: EFW ≈ 1,750 g, essentially the median for 31 weeks (1,751 g).
Frequently asked questions
How accurate is a scan weight?
Within ±15% in about 80% of cases. Near term a 3.5 kg estimate could easily be 3.0–4.0 kg at birth.
Which percentile chart is used?
A Hadlock in-utero reference. Local or customised charts (e.g. INTERGROWTH-21st) may place the same weight a few percentiles differently.
Related calculators
Educational estimate only — your midwife, doctor or paediatrician will confirm dates and advice for your pregnancy or baby.