Abdominal Circumference (AC) Calculator

Understanding Fetal Abdominal Circumference

The Abdominal Circumference Calculator compares a fetal AC measurement against Hadlock (1984) reference values for gestational weeks 14–40. It reports the percentile band, deviation from the median (p50), and the normal range (p10–p90) for the selected gestational age. Healthcare providers use AC to monitor fetal development, identify potential growth concerns such as fetal growth restriction (FGR) or macrosomia, and guide management decisions. A measurement in the 25th–75th percentile band is considered normal for a healthy pregnancy.

Why Fetal Growth Assessment Matters

Assessing fetal growth is paramount for ensuring optimal pregnancy outcomes. The abdominal circumference is highly sensitive to fetal nutrition and oxygenation, making it an excellent proxy for overall fetal size and weight. Deviations from the expected growth trajectory can signal underlying issues that require medical attention. For instance, a fetus consistently measuring below the 10th percentile may face increased risks of preterm birth or developmental challenges, while measurements above the 90th percentile could indicate gestational diabetes or delivery complications. Understanding these trends allows healthcare providers to intervene early, optimizing care for both mother and baby.

The Logic Behind AC Growth Assessment

The calculator uses a reference table based on Hadlock et al. (1984) — a widely used clinical standard that provides population percentiles (p5, p10, p25, p50, p75, p90, p95) for fetal abdominal circumference at each gestational week from 14 to 40.

The core logic works as follows:

1. Look up reference values for the selected gestational week
2. Compare your AC measurement against the percentile thresholds (p5, p10, p25, p75, p90, p95)
3. Assign a percentile band based on where your measurement falls
4. Calculate deviation from the median (p50):

Deviation = Measured AC - p50
Deviation % = (Deviation / p50) × 100

The percentile bands are:

• Below 5th: Significantly Small — possible FGR
• 5th – 10th: Small for GA — monitor closely
• 10th – 25th: Low-Normal — within acceptable range
• 25th – 75th: Normal for gestational age
• 75th – 90th: High-Normal — within acceptable range
• 90th – 95th: Large for GA — discuss with provider
• Above 95th: Significantly Large — possible macrosomia

Assessing Fetal Growth with an Example

Consider a pregnant individual at 30 weeks gestation whose ultrasound indicates a fetal abdominal circumference (AC) of 290 mm. We can use the calculator to assess this measurement.

1. Look up Week 30 reference values: p10 = 251 mm, p50 = 279 mm, p90 = 305 mm.
2. Determine percentile band: 290 mm is between p25 (265 mm) and p75 (293 mm), so the band is 25th – 75th (Normal).
3. Calculate deviation from median: 290 - 279 = +11.0 mm, which is 3.9% above median.
4. Normal range (p10–p90): 251–305 mm — the measurement of 290 mm is well within this range.
5. Clinical Guidance: Within normal limits — no immediate concern indicated.

This confirms that an AC of 290 mm at 30 weeks is normal for gestational age.

Manual Calculation Walkthrough

Let's walk through a second example: a fetus at 28 weeks gestation with an abdominal circumference of 260 mm.

1. Look up Week 28 reference values from the Hadlock table:

   • p5 = 222, p10 = 230, p25 = 243, p50 = 256, p75 = 269, p90 = 281, p95 = 289 (all in mm).

2. Determine the percentile band: 260 mm is greater than or equal to p25 (243) and less than p75 (269), so it falls in the 25th – 75th (Normal) band.

3. Calculate deviation from the median (p50): Deviation = 260 - 256 = +4.0 mm Deviation % = (4 / 256) × 100 = 1.6% above median

4. Report the normal range: p10–p90 = 230–281 mm. The measurement of 260 mm is comfortably within this window.

5. Clinical Guidance: Since the band includes "Normal," the result is "Within normal limits" with no immediate concern indicated.

This manual walkthrough confirms the calculator's output and demonstrates how the Hadlock reference table is used to assess fetal growth.

The History Behind Abdominal Circumference

The use of fetal biometry, including abdominal circumference (AC), in ultrasound dates back to the 1970s. Dr. Stuart Campbell at Queen Charlotte's Hospital in London pioneered the development and standardization of ultrasound measurements for fetal assessment. In 1984, Hadlock et al. published comprehensive reference tables for fetal biometric parameters including AC, establishing population-based percentile charts that remain widely used today. These charts allow clinicians to quantitatively assess fetal size and identify growth abnormalities by comparing individual measurements against established population norms. The AC measurement became a standard practice because the fetal abdomen — primarily the liver — is highly susceptible to changes in nutrient supply, making it an excellent indicator of fetal nutritional status and overall growth.