Health, Fitness, Diet & Pharmacology

Fetal Abdominal Circumference Percentile Calculator

Evaluate fetal growth by calculating the abdominal circumference percentile against standard gestational age growth curves.

wks
days
mm
Percentile
100
InterpretationLarge for Gestational Age (LGA)
Z-Score7.87
Expected Mean AC87.8 mm

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The Diagnostic Value of Fetal AC

In maternal-fetal medicine, Fetal Abdominal Circumference (AC) is one of the most critical biometric parameters measured during a prenatal ultrasound. Unlike head circumference (HC) or femur length (FL), the fetal AC is highly sensitive to the nutritional status of the fetus.

Because the fetal liver represents a large portion of the abdominal volume, and glycogen (sugar) is stored in the liver, fluctuations in fetal nutrition directly impact liver size—and consequently, the abdominal circumference.

How to Interpret the Percentile

The calculator uses standard regression curves to output a percentile based on the input Gestational Age (GA) and the measured AC.

  • 10th to 90th Percentile: Considered the normal range for fetal growth.
  • Below the 10th Percentile: May indicate Fetal Growth Restriction (FGR) or Small for Gestational Age (SGA). This often prompts closer surveillance, Doppler ultrasound studies, or evaluation of placental function.
  • Above the 90th Percentile: May indicate Macrosomia or Large for Gestational Age (LGA). This is frequently associated with maternal gestational diabetes and can impact delivery planning.

Z=Observed AC−Expected Mean ACStandard Deviation\begin{aligned} Z = \frac{\text{Observed AC} - \text{Expected Mean AC}}{\text{Standard Deviation}} \end{aligned}

Where:
Z=
Standard deviations from the mean
Observed AC=
The ultrasound measurement in millimeters
Expected Mean AC=
The expected average size for the gestational age

Quick Example: Calculating the Percentile

If a patient presents at 20 weeks and 0 days gestation with an ultrasound measurement of 150 mm for the AC:

  1. Expected Mean: The established mean AC for 20 weeks is approximately 149.6 mm.
  2. Standard Deviation: The standard deviation at this stage is approximately 13.5 mm.
  3. Z-Score: The observed measurement (150 mm) is just slightly above the mean, yielding a Z-score near +0.03.
  4. Percentile: This corresponds to the 51st Percentile, indicating perfectly average, normal growth.

The Mathematical Framework

This calculator utilizes an approximation of the landmark Hadlock (1984) polynomial regression model. While many modern health authorities (such as the WHO Fetal Growth Standards and the INTERGROWTH-21st Project) offer updated references that dynamically adjust the variance across the pregnancy, the Hadlock equations remain an entrenched, global standard integrated into almost all modern ultrasound reporting software.

The tool computes the predicted mean utilizing the formula: Mean AC (cm) = -8.74 + 1.04 × GA - 0.0082 × GA². It then estimates the standard deviation (assuming an approximate 9% coefficient of variation) to determine the exact Z-score and percentile rank.

Frequently Asked Questions

Most modern obstetric ultrasound machines output biometric measurements in millimeters (mm) by default. This calculator expects input in millimeters to ensure maximum precision.

A slowing or 'dropping' AC percentile trajectory in the third trimester is a classic warning sign of asymmetrical Fetal Growth Restriction (FGR). It indicates the fetus is actively shunting blood and nutrients away from the abdomen (liver) toward critical organs like the brain, often due to placental insufficiency.

While the WHO Fetal Growth Standards (published in 2017) are rigorously designed for an international population, the Hadlock curves from the 1980s remain the default in the vast majority of clinical ultrasound machines. Always refer to your specific clinic's standard operating procedures.

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