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Fetal Abdominal Circumference (AC) Calculator

Estimate gestational age and fetal weight using Hadlock regression formulas. Includes reverse due-date calculation and percentile lookups.

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Measurements (mm)

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About

Fetal biometry relies heavily on Abdominal Circumference (AC) to assess fetal size and nutritional status. Unlike skeletal measurements (femur length, biparietal diameter), AC is sensitive to soft tissue mass, making it the primary marker for detecting intrauterine growth restriction (IUGR) or macrosomia. In clinical practice, accurate dating requires correlating millimeter measurements against established nomograms.

This tool utilizes the Hadlock regression models to estimate Gestational Age (GA) and Estimated Fetal Weight (EFW). It accounts for standard deviations to provide a clinical range rather than a misleadingly precise single date. The logic incorporates polynomial equations derived from cross-sectional data of fetal development.

sonography gestational age fetal weight pregnancy ultrasound hadlock

Formulas

The core estimation relies on the Hadlock log-linear regression model. The relationship between Abdominal Circumference and Gestational Age is non-linear:

lnGA ≈ 1.54 + 0.042×√AC + 0.00003×AC2

For Estimated Fetal Weight (EFW), a combination of AC, BPD (Biparietal Diameter), and FL (Femur Length) yields the highest accuracy:

log10EFW = 1.335 − 0.0034×AC×FL + 0.0316×BPD + 0.0457×AC + 0.1623×FL

Reference Data

AC (mm)Mean GA (Weeks)5th Percentile95th PercentileGrowth Rate (mm/week)
10016w 3d15w 0d17w 6d11.5
15020w 5d19w 1d22w 2d10.8
20024w 6d23w 1d26w 4d10.1
25029w 3d27w 3d31w 2d9.4
30034w 1d32w 0d36w 3d8.7
32036w 1d33w 6d38w 2d8.2
34038w 2d35w 5d40w 4d7.5
36040w 5d37w 6d42w 6d6.9

Frequently Asked Questions

In early pregnancy, fetal growth is largely determined by genetic hyperplasia (cell division), resulting in uniform measurements. By the third trimester, hypertrophy (cell growth) dependent on placental function and nutrition takes over. This biological variance means an AC of 300mm has a wider standard deviation (±3 weeks) than an AC of 100mm (±1 week).
Standard ultrasound dating works by measuring anatomy to find the age. Reverse calculation takes a known Due Date (EDD) or Last Menstrual Period (LMP) to calculate the expected gestational age today, and then outputs the expected AC measurement range for that specific day. This helps identify if a fetus is measuring "small for dates" or "large for dates".
Technically, yes, using Campbell & Wilkin's formulas, but the error margin is significantly higher. The Hadlock 3-parameter formula (AC, BPD, FL) reduces the standard error of estimation to approximately 15%, whereas AC alone can vary by over 20% due to fetal positioning and abdominal compression.