Fetal Growth Percentile Calculator
Estimates fetal weight from ultrasound biometry (BPD, HC, AC, FL) with the Hadlock four-parameter formula, for comparison against gestational-age percentile charts. Used by clinicians and expectant parents to detect growth restriction or macrosomia.
Last updated: September 2026
Formula below · 3 sources (CDC, acog.org, mayoclinic.org) · Updated Sep 2026
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About this calculator
This calculator applies the Hadlock (1985) formula to estimate fetal weight from four ultrasound measurements. The formula works in base-10 log space: log10(EFW) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL, where BPD is biparietal diameter, HC is head circumference, AC is abdominal circumference, and FL is femur length, all in centimetres (the calculator converts your mm entries). Raising 10 to that power gives the Estimated Fetal Weight (EFW) in grams. The calculator does not ask for gestational age, so it does not itself assign a percentile; your provider plots the EFW on a reference chart for your exact gestational week. A result between the 10th and 90th percentile is considered appropriate for gestational age (AGA); below 10th may indicate intrauterine growth restriction (IUGR), while above 90th may indicate macrosomia.
How to use
Suppose an ultrasound at 32 weeks shows BPD = 82 mm, HC = 295 mm, AC = 278 mm, FL = 63 mm. Step 1 — convert to cm: BPD 8.2, HC 29.5, AC 27.8, FL 6.3. Step 2 — apply the formula: log10(EFW) = 1.3596 − 0.00386(27.8)(6.3) + 0.0064(29.5) + 0.00061(8.2)(27.8) + 0.0424(27.8) + 0.174(6.3) = 1.3596 − 0.676 + 0.189 + 0.139 + 1.179 + 1.096 ≈ 3.286. Step 3 — EFW = 10^3.286 ≈ 1,933 g. Step 4 — compare with a reference chart: the Hadlock 50th percentile at 32 weeks is about 1,950 g, so this estimate is close to average.
Frequently asked questions
What is the Hadlock formula and why is it used to estimate fetal weight?
The Hadlock formula is a regression equation derived from a large population of ultrasound measurements published by Hadlock et al. in 1985. It combines four biometric parameters — BPD, HC, AC, and FL — because no single measurement captures overall fetal size accurately. The multi-parameter approach significantly reduces estimation error compared to using AC alone. It remains the most widely validated and clinically adopted EFW formula worldwide.
How accurate is fetal weight estimation from ultrasound measurements?
Ultrasound-based EFW has an inherent margin of error of roughly ±15–20% of actual birth weight, even with the best technique. Accuracy decreases late in pregnancy, in obese patients, or when fetal position limits image quality. A single measurement should therefore be interpreted alongside growth trends over serial scans rather than in isolation. Clinical decisions about early delivery or intervention are never made on EFW alone.
What percentile range is considered normal for fetal growth?
A fetus whose EFW falls between the 10th and 90th percentile for gestational age is classified as appropriate for gestational age (AGA). An EFW below the 10th percentile raises concern for fetal growth restriction (FGR or IUGR), while an EFW above the 90th percentile suggests large for gestational age (LGA) or macrosomia. These thresholds are guidelines; your obstetrician will consider the full clinical picture, including growth velocity, Doppler findings, and amniotic fluid levels, before drawing conclusions.