Baby Weight Calculator
Estimating Weight Before Birth
Fetal weight can't be measured directly, so obstetric ultrasound instead measures four bony and soft-tissue landmarks — the head, the abdomen, and the thigh bone — and runs them through a regression formula built from thousands of scanned pregnancies. The Hadlock formula, published in the 1980s, remains the most widely used method for converting those four measurements into an estimated weight.
The Formula
log10(Weight in g) = 1.3596 + 0.0064×HC + 0.0424×AC + 0.174×FL + 0.00061×BPD×AC − 0.00386×AC×FL
HC is head circumference, BPD is biparietal diameter, AC is abdominal circumference, and FL is femur length, all measured in centimeters from the ultrasound scan. The formula outputs a base-10 logarithm of weight in grams, which is then converted back with 10 raised to that power.
Reading the Result
Because the formula is a statistical fit rather than a direct measurement, published accuracy studies put the typical estimate within about 10–15% of actual birth weight — closer for term pregnancies scanned recently, wider for early or unusually sized fetuses.
Standard Birth Weight Classifications
| Weight | Classification |
|---|---|
| Below 2,500 g | Low birth weight |
| 2,500 g – 4,000 g | Normal birth weight |
| Above 4,000 g | High birth weight (macrosomia) |
These ranges describe birth weight at delivery, not mid-pregnancy ultrasound estimates, which naturally fall below the eventual birth weight the further out from delivery they're taken.
How to Use This Calculator
- Enter the Head Circumference (HC) from your ultrasound report, in centimeters.
- Enter the Biparietal Diameter (BPD), in centimeters.
- Enter the Abdominal Circumference (AC), in centimeters.
- Enter the Femur Length (FL), in centimeters.
- Select Calculate to see the estimated fetal weight in grams and pounds/ounces.
Related Calculations
Once your baby has arrived, the Baby Height Percentile Calculator tracks growth against standard reference charts.