Growth Ultrasound Calculator: What It Can Estimate

A growth ultrasound calculator estimates a fetus’s size from measurements taken during an obstetric ultrasound, usually producing an estimated fetal weight (EFW) and a percentile for gestational age. Entering measurements into a formula can demonstrate how the arithmetic works, but it cannot diagnose fetal growth or replace the estimate and interpretation provided by a qualified clinician.

Formula

This example uses the Hadlock-3 equation, a published formula that estimates fetal weight from head circumference, abdominal circumference, and femur length:

log₁₀(EFW) = 1.326 − 0.00326 × AC × FL + 0.0107 × HC + 0.0438 × AC + 0.158 × FL

EFW is estimated fetal weight in grams; HC is head circumference in centimeters; AC is abdominal circumference in centimeters; and FL is femur length in centimeters. The calculator accepts the measurements in millimeters, so the expression uses the equivalent coefficients and units directly. The equation returns the base-10 logarithm of weight; raising 10 to that result gives EFW in grams. The formula is an illustration, not a universal clinical standard: practices and calculators may use different equations, measurement combinations, or unit conventions.

Worked Example

With the calculator’s default inputs, HC = 300 mm, AC = 300 mm, and FL = 60 mm. These are 30 cm, 30 cm, and 6 cm in the conventional form of the equation.

1. Calculate the logarithm: 1.326 − (0.00326 × 30 × 6) + (0.0107 × 30) + (0.0438 × 30) + (0.158 × 6).
2. Evaluate the terms: 1.326 − 0.5868 + 0.321 + 1.314 + 0.948 = 3.3222.
3. Convert from the logarithm: 10^3.3222 ≈ 2,100 grams when rounded to the calculator’s whole-gram display.
4. Convert grams to kilograms: 2,100 ÷ 1,000 = 2.10 kg.

The displayed result is therefore approximately 2,100 g (2.10 kg). It is only an estimate generated from the entered numbers, not a measured birth weight or a conclusion about whether growth is normal.

What a Growth Ultrasound Measures

A fetal growth scan is an ultrasound examination in which a sonographer obtains biometric measurements. Common measurements include head circumference, abdominal circumference, and femur length; some approaches also use biparietal diameter, the width of the head. A formula combines selected measurements to estimate fetal weight. The report may also compare measurements with a reference chart for the pregnancy’s gestational age.

A calculator can help explain the relationship between measurements and an estimated weight, or let a reader check the arithmetic of a particular equation. It cannot interpret an entire scan. A clinical report may include image quality, measurement technique, prior scans, pregnancy dating, and other findings that a simple equation does not contain.

How to Use the Result

Use the same units the calculator requests. This example requires millimeters; many clinical equations are presented with centimeters, and mixing the two changes the result substantially. If you transcribe values from a report, check each label and unit rather than assuming all numbers are in the same unit.

• Enter the biometric measurements from the same scan and confirm which measurement each number represents.
• Check that the calculator identifies its equation and units. Different equations can give different estimates from the same scan.
• Read the output as an approximate weight in grams or kilograms, not as a promise of the baby’s actual weight.
• Use the clinician’s report for gestational-age percentile and interpretation; this calculator does not calculate a percentile.

One gram is 0.001 kilogram, so 2,100 g equals 2.10 kg. In US customary weight units, that is about 4.6 lb, but converting the estimate to pounds does not make it more precise. The calculator rounds its gram output to a whole number; the underlying uncertainty is much larger than a one-gram distinction.

Why Estimates Differ

Ultrasound weight is inferred from body dimensions, not obtained by weighing the fetus. Small differences in placing measurement calipers, fetal position, image quality, or the equation selected can affect the result. Estimates also have inherent uncertainty, and a later scan may produce a different number because the fetus has grown as well as because measurements vary.

A common misconception is that an EFW is an exact current weight or a reliable prediction of birth weight. It is neither. A percentile is also not the same thing as a diagnosis: it describes a position relative to a reference population at a specified gestational age. Clinicians consider the full clinical context and, when useful, trends across scans rather than treating a single calculator output as definitive.

Limits and Clinical Context

This calculator requires only three inputs and demonstrates one formula. Clinical reports may use another Hadlock version or a different accepted method; some use additional head measurements. The result here should not be compared uncritically with a report generated by a different equation. Nor can this tool establish gestational age, assess amniotic fluid or placental function, diagnose a growth condition, or determine what care is appropriate.

Ask the clinician who ordered or performed the scan to explain the reported EFW, percentile, and any follow-up plan. If the scan report contains a value that seems inconsistent with this example, first check the units, equation, and measurements used; a difference alone does not show that either result is wrong. Medical decisions should be based on the complete evaluation, not a web calculation.

Frequently Asked Questions

What does a growth ultrasound calculator calculate?
It applies a specified equation to fetal biometric measurements to produce an estimated fetal weight. Some tools also calculate a percentile, but this one does not.

Is estimated fetal weight the baby’s exact weight?
No. It is a formula-based estimate from ultrasound dimensions. Measurement variation and the formula itself mean the actual weight can differ.

Why does my calculator result differ from my ultrasound report?
The report may use a different equation, different measurements, or different units. Confirm those details before comparing numbers, and ask the clinical team to explain the report.

Can I calculate a fetal growth percentile with this tool?
No. A percentile needs a gestational age and a specific reference chart or method. The weight estimate alone is not enough to determine one.

Can this calculator tell whether fetal growth is normal?
No. It cannot diagnose a growth problem or interpret a scan. A qualified clinician must assess the estimate alongside dating, scan findings, and the broader pregnancy context.

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in hc | Head circumference | mm | 300 | 0
in ac | Abdominal circumference | mm | 300 | 0
in fl | Femur length | mm | 60 | 0
out efw | Estimated fetal weight | g | 10^(1.326 - 0.00326 * ac * fl + 0.0107 * hc + 0.0438 * ac + 0.158 * fl) | 0
out kg | Estimated fetal weight | kg | efw / 1000 | 2