Fetal Weight Calculator
How big is your baby right now?
Based on Hadlock-4 (1985) formula. ±10–15% error vs actual birth weight. Learn more
For reference only. See your OB-GYN for an accurate assessment.
How is it calculated?
Estimated fetal weight (EFW) is computed with the Hadlock-4 formula (1985). Four measurements from an obstetric ultrasound feed into a log-regression equation:
- BPD — biparietal diameter
- HC — head circumference
- AC — abdominal circumference
- FL — femur length
Hadlock derived the equation from 392 fetuses, and it is still the default EFW formula used by most ultrasound systems and OB-GYN practices worldwide.
Limitations of this estimate
The Hadlock-4 EFW carries a ±10-15% error versus actual birth weight, and the error grows in these situations:
- Very low (oligohydramnios) or very high (polyhydramnios) amniotic fluid blurs the measurement itself.
- Higher maternal BMI — subcutaneous fat reduces ultrasound resolution.
- Breech or transverse fetal position distorts the BPD/HC reads.
- Sonographer skill varies; the same fetus can yield different numbers.
- Reliability drops at both extremes — macrosomia (EFW > 4 kg) and low birth weight (EFW < 2.5 kg).
Repeat measurements at 1-2 week intervals are recommended. The most accurate weight is the actual birth weight at delivery.
Sources
Frequently asked
Where do the BPD/HC/AC/FL numbers come from?
From your obstetric ultrasound (anomaly scan / growth scan) report — values are recorded in millimetres. From about 28 weeks the EFW is usually printed on the report too, but if you plug the same measurements into this calculator you'll get the same number.
Without those measurements this tool isn't useful, so see your OB-GYN first.
Is one measurement enough?
A single EFW has ±10-15% error and varies by sonographer.
For macrosomia work-ups or intrauterine growth restriction (IUGR), the standard is repeat measurement at 1-2 week intervals to watch the trend. A diagnosis is never based on a single time point.