Hadlock Fetal Weight Calculator
Enter the four biometric measurements from an ultrasound report to estimate fetal weight with the Hadlock IV equation. The result also shows how much the estimate moves when each measurement is shifted by a small tolerance.
Estimated fetal weight1,925
- Estimated weight in kilograms
- 1.92
- Estimated weight in pounds and ounces
- 4 lb 3.9 oz
- Lower sensitivity estimate
- 1,875
- Upper sensitivity estimate
- 1,975
- Sensitivity-band width
- 5.2%
- Most influential measurement
- Femur length (FL), about 1.53% per entered tolerance
Hadlock IV requires BPD, HC, AC, and FL from the same fetus and the same ultrasound exam.
For twins or higher-order multiples, calculate each fetus separately.
How to use this calculator
- Enter BPD, HC, AC, and FL exactly as they appear on the ultrasound report, in millimeters.
- Leave the tolerance at 1 mm or enter the measurement change you want to test.
- Read the estimated fetal weight in grams, kilograms, and pounds with ounces.
- Use the sensitivity band to see how much small measurement differences can move the formula result.
How the Hadlock IV estimate is calculated
The Hadlock IV equation estimates fetal weight from four ultrasound measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). Enter the report values in millimeters. The calculator converts each value to centimeters before using the equation.
The log equation is:
L = 1.3596 + 0.0064 * HC + 0.0424 * AC + 0.174 * FL + 0.00061 * BPD * AC - 0.00386 * AC * FL
The estimated fetal weight in grams is then:
EFW = 10^L
The calculator rounds the main result to the nearest gram, divides by 1,000 for kilograms, and converts grams to pounds and ounces using 28.3495 grams per ounce.
What the sensitivity band means
Ultrasound measurements are not perfectly fixed numbers. Caliper placement, fetal position, image quality, and operator technique can move a reported measurement by a small amount. The tolerance field lets you choose a simple mathematical shift, such as 1 mm, and see how the equation responds.
The lower sensitivity estimate subtracts that tolerance from all four measurements. The upper sensitivity estimate adds it to all four measurements. The band width is the difference between those two estimates divided by the central estimate. This is not a clinical confidence interval; it is only a formula sensitivity check.
Which measurement moves the result most
Hadlock IV includes direct terms for HC, AC, and FL plus interaction terms between BPD and AC, and between AC and FL. Because of those interactions, the most influential measurement can change with the entered values. The calculator tests each measurement one at a time by adding and subtracting the selected tolerance, then reports the measurement that creates the largest percentage change.
What this calculator leaves out
The estimate does not account for gestational age, fetal sex, population differences, image quality, fetal position, growth pattern, maternal conditions, or whether the fetus is part of a multiple pregnancy. For twins or higher-order multiples, calculate each fetus separately from that fetus's own measurements. The result also cannot diagnose fetal growth restriction, macrosomia, or any pregnancy condition on its own.
Worked example
Suppose an ultrasound report lists BPD 82 mm, HC 295 mm, AC 280 mm, and FL 62 mm, with a 1 mm tolerance. In centimeters, those inputs are BPD 8.2, HC 29.5, AC 28.0, and FL 6.2. The Hadlock IV equation gives L = 3.28436, so 10^L = 1,924.686 grams.
The displayed estimate is 1,925 g, or 1.92 kg, which is about 4 lb 3.9 oz. With all four measurements shifted down and up by 1 mm, the sensitivity estimates are 1,875 g and 1,975 g. The band width is about 5.20%, and femur length is the most influential measurement for this set of inputs.
Common questions
What are BPD, HC, AC, and FL on an ultrasound report?
BPD is biparietal diameter, a side-to-side head measurement. HC is head circumference, AC is abdominal circumference, and FL is femur length. The Hadlock IV equation requires all four measurements from the same fetus.
How does the Hadlock IV formula estimate fetal weight?
The formula converts BPD, HC, AC, and FL to centimeters and combines them in a logarithmic equation. The calculator raises 10 to that result to get estimated fetal weight in grams. The equation is a mathematical estimate, not a direct weighing of the fetus.
How accurate is ultrasound estimated fetal weight?
Ultrasound estimated fetal weight can differ from actual birth weight by a meaningful amount, especially near the extremes of fetal size. The sensitivity band here only shows how the Hadlock IV equation changes when measurements shift by the selected tolerance. It does not replace the uncertainty assessment made by a clinician or sonographer.
Should measurements be entered in millimeters or centimeters?
Enter the numbers in millimeters, because most ultrasound reports list BPD, HC, AC, and FL that way. The calculator converts millimeters to centimeters internally before applying the formula. If you enter centimeter values by mistake, the estimate will be far too small.
Can this calculator be used for twins?
Yes, but calculate each fetus separately. Use fetus A measurements for one run and fetus B measurements for another run. Do not mix measurements from different fetuses in the same calculation.
Does estimated fetal weight diagnose fetal growth restriction?
No. Estimated fetal weight is one piece of information that clinicians interpret with gestational age, growth trend, Doppler findings, amniotic fluid, maternal history, and other clinical context. Do not use this result alone to diagnose fetal growth restriction, macrosomia, or another pregnancy condition.