SoupCalc

Fetal Weight Calculator – Hadlock 1985 Formula

What this tool estimates

This tool produces an estimated fetal weight (EFW) in grams using the Hadlock 1985 four-parameter regression formula. The estimate is derived from four standard ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). All four inputs are entered in millimetres and automatically converted to centimetres before the equation is applied.

The result is a single weight estimate in grams. It does not assign a gestational-age percentile, compare the fetus to a growth chart, or classify the result as normal, small, or large for gestational age. Those interpretations require additional clinical context that this tool does not provide.

The Hadlock 1985 equation

The formula used in this tool was published by Hadlock and colleagues in 1985. It is one of the most widely cited and clinically validated models for sonographic estimation of fetal weight. The equation is a log-transformed linear regression that combines the four biometric parameters with interaction terms:

log₁₀(EFW) = 1.3596 + 0.0064 × HC + 0.0424 × AC + 0.174 × FL + 0.00061 × BPD × AC − 0.00386 × AC × FL

All measurements are in centimetres. The estimated fetal weight in grams is then obtained by raising 10 to the power of the result:

EFW (g) = 10^(log₁₀(EFW))

This equation acknowledges that no single fetal dimension captures overall size well. The interaction terms — BPD × AC and AC × FL — reflect the reality that head size, abdominal girth, and limb length influence weight in combination, not in isolation.

Required inputs

The tool requires four measurements, each obtained during a standard second- or third-trimester ultrasound examination:

  • BPD (Biparietal Diameter) — the transverse diameter of the fetal skull at the level of the thalami, measured from outer edge to inner edge, in millimetres.
  • HC (Head Circumference) — the perimeter of the fetal head measured in the same axial plane as the BPD, in millimetres.
  • AC (Abdominal Circumference) — the perimeter of the fetal abdomen measured in a transverse plane at the level of the stomach and umbilical vein, in millimetres.
  • FL (Femur Length) — the length of the ossified diaphysis of the femur, in millimetres.

All four measurements should be obtained during the same ultrasound examination to ensure anatomical consistency. The tool converts each value from millimetres to centimetres by dividing by 10 before substituting into the equation.

Worked example

Consider a routine third-trimester scan with the following measurements:

  • BPD: 80 mm (8.0 cm)
  • HC: 300 mm (30.0 cm)
  • AC: 280 mm (28.0 cm)
  • FL: 60 mm (6.0 cm)

Substituting into the Hadlock 1985 equation:

log₁₀(EFW) = 1.3596 + 0.0064 × 30.0 + 0.0424 × 28.0 + 0.174 × 6.0 + 0.00061 × 8.0 × 28.0 − 0.00386 × 28.0 × 6.0

Breaking this down step by step:

  • 0.0064 × 30.0 = 0.192
  • 0.0424 × 28.0 = 1.1872
  • 0.174 × 6.0 = 1.044
  • 0.00061 × 8.0 × 28.0 = 0.13664
  • 0.00386 × 28.0 × 6.0 = 0.64848

Summing the terms: 1.3596 + 0.192 + 1.1872 + 1.044 + 0.13664 − 0.64848 = 3.27096

Raising 10 to this power: 10^3.27096 ≈ 1,866 grams.

The estimated fetal weight is therefore approximately 1,866 g, or about 1.87 kg. This example illustrates how the cross-product terms moderate the raw contribution of AC and FL, preventing the abdominal circumference term from unduly dominating the estimate.

How to interpret the estimate

The number this tool returns is an estimate, not a measurement. It is derived from a statistical model fitted to a population of pregnancies, and it represents the most probable weight given the four biometric inputs under the assumptions of that model. The result should be treated as one data point among many in a prenatal assessment.

Only a qualified prenatal care professional — such as an obstetrician, maternal-fetal medicine specialist, or sonographer — can interpret fetal growth trends appropriately. Weight estimates gain meaning when plotted serially over time, compared against population-appropriate growth curves, and evaluated alongside Doppler studies, amniotic fluid indices, and maternal health factors. A single EFW value, taken in isolation, does not constitute a diagnosis and should not be used to make clinical decisions.

This tool does not provide medical advice, does not recommend any course of action, and is not a substitute for professional prenatal care. If you have concerns about fetal growth or your pregnancy, speak with your healthcare provider.

Accuracy and limitations

The Hadlock 1985 formula carries a reported mean absolute percentage error typically in the range of 7 to 10 percent when compared against birth weight within 48 hours of delivery. This means that for a fetus with an estimated weight of 2,000 g, the actual birth weight may reasonably fall between roughly 1,800 g and 2,200 g — and sometimes outside that range.

Several factors contribute to this error. Ultrasound measurement technique introduces variability: even experienced sonographers can obtain slightly different caliper placements on the same fetus, and small differences in AC measurement have a disproportionately large effect on the final weight estimate because AC is the most heavily weighted term. Fetal position, maternal body habitus, and amniotic fluid volume can all degrade image quality and measurement accuracy.

The model was developed from a specific population of pregnancies in the United States in the mid-1980s. It may perform differently in contemporary populations, in pregnancies complicated by conditions such as gestational diabetes or intrauterine growth restriction, at extremes of fetal weight, and in populations with different ethnic or demographic profiles. The equation does not account for fetal sex, maternal height or weight, parity, or gestational age — all of which can influence birth weight independently of the biometric measurements.

This tool reports a single weight estimate. It does not calculate a gestational-age percentile, confidence interval, or prediction interval. Without a percentile, it is not possible to assess whether the estimated weight is appropriate for gestational age. Clinical decisions about timing of delivery, mode of delivery, or surveillance frequency should never rest on a single EFW value alone.

Sources

Editorial record

This article was written by the SoupCalc Editorial Team to provide transparent documentation of the Hadlock 1985 fetal weight estimation formula and its clinical context. It draws on the original peer-reviewed publication and on public guidance from the American College of Obstetricians and Gynecologists. The content is reviewed periodically for accuracy and to reflect any relevant updates to clinical guidance. Author: SoupCalc Editorial Team Last reviewed: August 11, 2026.