Estimate fetal weight using the Hadlock formula from ultrasound biometry. Shows EFW plus an approximate gestational-age percentile reference.
The Fetal Weight Percentile Calculator estimates fetal weight from standard ultrasound biometry using the Hadlock formula. It combines biparietal diameter, head circumference, abdominal circumference, and femur length, then compares the result with a gestational-age reference range.
That gives you an estimated fetal weight, an approximate percentile, and a basic classification such as small for gestational age, appropriate for gestational age, or large for gestational age. The percentile view is useful because it puts a single weight estimate into the context of expected growth at the same gestational age.
The page is aimed at quick growth screening, not at replacing a full obstetric assessment or serial trend review.
Percentile-based fetal weight estimates help make growth assessment easier to compare across visits and across pregnancies. They are most useful as a screening view that points to possible concern and suggests when closer follow-up or a more complete assessment may be worth considering.
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.
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This page uses a two-step estimate: it first calculates estimated fetal weight from Hadlock biometry, then places that estimate against a simplified gestational-age reference curve to generate an approximate percentile and SGA/AGA/LGA band. It is intended as a quick screening summary rather than a formal fetal-growth-standard implementation.
Because the percentile layer is approximate, borderline results should not be treated as interchangeable with a published Hadlock, INTERGROWTH, or customized growth chart. Serial growth trend, Doppler findings, fluid status, and the obstetric service's chosen fetal-growth standard remain more important than any one point estimate.
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.
Sometimes. Different services use different fetal-growth standards or customized charts, and that can change percentile classification. This page is best used as a quick screening estimate rather than as a substitute for the exact growth standard used in your obstetric service.