Weighing a baby before birth
Nobody can put an unborn baby on a scale, but ultrasound can measure its head, abdomen and thigh bone very precisely. Researchers compared these measurements with weights at birth and derived formulas that turn them into an estimated fetal weight (EFW). The formulas published by Frank Hadlock and colleagues in 1985 remain the most widely used and are built into most ultrasound machines.
The measurements
- BPD (biparietal diameter): the width of the head between the two parietal bones.
- HC (head circumference).
- AC (abdominal circumference): measured at the level of the liver and stomach; it reflects the baby’s fat and liver size and matters most for weight.
- FL (femur length): the length of the thigh bone.
The Hadlock formulas
With all measurements in centimeters and the weight in grams:
- Four parameters: log₁₀ EFW = 1.3596 − 0.00386 AC × FL + 0.0064 HC + 0.00061 BPD × AC + 0.0424 AC + 0.174 FL
- HC, AC and FL: log₁₀ EFW = 1.326 − 0.00326 AC × FL + 0.0107 HC + 0.0438 AC + 0.158 FL
- AC and FL only: log₁₀ EFW = 1.304 + 0.05281 AC + 0.1938 FL − 0.004 AC × FL
Worked example
At a third-trimester scan, the measurements are BPD 82 mm, HC 295 mm, AC 280 mm and FL 62 mm:
- Four-parameter estimate: 1,925 g, about 4.24 lb (4 lb 4 oz).
- HC-AC-FL formula: 1,913 g; AC-FL formula: 1,949 g. The three agree within 2%.
- Usual error: about ±15%, or ±289 g, so the true weight is likely between about 3.6 and 4.9 lb.
- FL/AC ratio: 22.1%, within the usual 20 to 24%.
How accurate is it?
On average, ultrasound estimates fall within about 10% of the birth weight, but individual errors of 15 to 20% occur, especially for very large babies, very small babies, late in pregnancy, and when the mother has a high BMI or little amniotic fluid. Estimates are most useful when repeated a few weeks apart to see the growth trend.
What doctors look at
- Percentile for gestational age: below the 10th percentile, the baby is small for gestational age, which may prompt Doppler checks of blood flow and closer monitoring.
- Above the 90th percentile or above about 4,000 g (8 lb 13 oz) near term: large for gestational age, which may change plans for delivery.
- Body proportions: a small abdomen compared with the head can suggest the baby is saving resources for the brain.
Only your obstetrician or midwife can interpret an estimate with the dates, previous scans and your own health. Ask them about any number that worries you.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.