Estimate fetal weight using Hadlock formula from ultrasound biometry. Calculates EFW percentile, z-score, and classifies SGA/AGA/LGA.
The Fetal Weight Percentile Calculator uses the Hadlock (1985) formula to estimate fetal weight from four standard ultrasound biometric measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The estimated fetal weight (EFW) is then plotted against gestational age-specific reference ranges to determine the growth percentile.
Fetuses below the 10th percentile are classified as small for gestational age (SGA), which may indicate fetal growth restriction (FGR) requiring surveillance with Doppler velocimetry, amniotic fluid assessment, and serial ultrasound growth measurements. Those above the 90th percentile (large for gestational age, LGA) prompt screening for gestational diabetes and assessment of macrosomia risk for delivery planning.
The Hadlock formula remains the most widely used EFW method, with a systematic error of approximately ±10-15%. This calculator provides percentile estimation, z-score, and classification to support prenatal care decision-making. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case.
Fetal growth assessment is one of the most important components of prenatal care. Undetected growth restriction is associated with stillbirth, neonatal morbidity, and long-term adverse outcomes. Conversely, macrosomia increases risk of shoulder dystocia, birth injury, and cesarean delivery.
Accurate EFW percentile calculation standardizes growth assessment across providers and facilities, enabling appropriate referral, surveillance intensity, and delivery timing decisions.
Hadlock EFW: log₁₀(EFW) = 1.3596 + 0.0064×HC + 0.0424×AC + 0.174×FL + 0.00061×BPD×AC − 0.00386×AC×FL (All biometry in cm; EFW in grams) SGA: <10th percentile AGA: 10th-90th percentile LGA: >90th percentile Severe FGR: <3rd percentile
Result: EFW ~1800g — 55th percentile (AGA)
With biometry measurements at 32 weeks producing an EFW of approximately 1800g, the fetus is at the 55th percentile — appropriate for gestational age with normal growth velocity.
Multiple Hadlock equations exist using different parameter combinations. The 4-parameter formula (BPD, HC, AC, FL) provides the best overall accuracy. When BPD is unreliable (dolichocephaly, engaged head), the 3-parameter HC+AC+FL formula is preferred. Select the appropriate formula based on measurement quality and fetal position.
Early-onset FGR (<32 weeks) is often associated with placental insufficiency and abnormal uterine artery Doppler waveforms. Management includes serial ultrasound, Doppler surveillance (umbilical artery, MCA, ductus venosus), and timed delivery. Late-onset FGR (≥32 weeks) may have normal Dopplers and requires cerebro-placental ratio (CPR) assessment for detection.
AI-assisted ultrasound biometry is showing promise for reducing inter-operator variability. Magnetic resonance imaging provides more accurate fetal weight estimation in obese patients and at the extremes of fetal weight but is not widely available for routine prenatal care.
The Hadlock formula has a systematic error of ±10-15% (95% CI). Accuracy decreases at extremes — very small and very large fetuses have greater prediction error. Adjacent measurements (e.g., 1800g ± 250g) represent a clinically significant range. Serial growth trend assessment is more informative than a single EFW.
AC is the single most predictive measurement for EFW (r² ~0.75). Combining AC with FL gives nearly as good accuracy as the 4-parameter Hadlock formula. HC and BPD add incremental accuracy but are affected by head shape variability (dolichocephaly).
For normal pregnancies, growth assessment at 28-32 weeks is standard. For suspected FGR, serial scans every 2-3 weeks monitor velocity. More frequent scanning (<2 weeks) may not show detectable growth changes due to measurement variability.
SGA (small for gestational age) is a statistical definition: EFW <10th percentile. FGR (fetal growth restriction) is a pathologic diagnosis suggesting the fetus has not reached its growth potential. Some SGA fetuses are constitutionally small but healthy; some AGA fetuses may be growth restricted if their genetically expected weight is higher.
LGA (>90th percentile) should prompt screening for gestational diabetes (OGTT if not done). EFW >4500g in diabetic mothers or >5000g in non-diabetic mothers is an indication to discuss planned cesarean due to shoulder dystocia risk.
Customized charts adjusted for maternal height, weight, ethnicity, and parity may improve detection of truly growth-restricted fetuses by reducing false-positive SGA classification of constitutionally small but healthy fetuses. GROW and Intergrowth-21st provide alternative references.