Fetal Growth Percentile Calculator
Estimates fetal weight from ultrasound biometry (BPD, HC, AC, FL) and plots it against gestational-age percentiles. Used by clinicians and expectant parents to detect growth restriction or macrosomia.
About this calculator
This calculator applies the Hadlock (1985) formula to estimate fetal weight from four ultrasound measurements. The formula works in log space: log(EFW) = 1.3596 + 0.064·ln(BPD) + 0.1495·ln(HC) + 0.8669·ln(AC) + 0.084·ln(FL), where BPD is biparietal diameter, HC is head circumference, AC is abdominal circumference, and FL is femur length, all in millimetres. Taking the natural exponent of that result gives the Estimated Fetal Weight (EFW) in grams. The EFW is then compared to published Hadlock reference curves for the entered gestational age to produce a percentile. 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 — take natural logs: ln(82) ≈ 4.407, ln(295) ≈ 5.688, ln(278) ≈ 5.628, ln(63) ≈ 4.143. Step 2 — apply the formula: log(EFW) = 1.3596 + 0.064(4.407) + 0.1495(5.688) + 0.8669(5.628) + 0.084(4.143) ≈ 7.688. Step 3 — exponentiate: EFW = e^7.688 ≈ 2,178 g. Step 4 — look up the 32-week reference table; 2,178 g falls near the 50th percentile, indicating normal fetal growth.
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.