Fetal Weight Percentile Calculator — Hadlock 1991 Standards
Enter the gestational age in weeks and the ultrasound estimated fetal weight in grams to get the fetal weight percentile using Hadlock 1991 normative standards, along with the SGA/AGA/LGA classification and the 10th and 90th percentile reference weights.
weeks
g
Appropriate for gestational age (AGA)
- 1
Median weight at this gestational age
1,702 gHadlock 1991 reference median, linearly interpolated between whole-week values. - 2
Log-normal z-score
(ln(1,702) − ln(1,702)) ÷ 0.12 = 0 - 3
Fetal weight percentile
Φ(0) × 100 = 50Φ is the standard normal CDF (Abramowitz & Stegun approximation).
How does this calculator work?
Using Hadlock 1991 log-normal standards, fetal weight percentile = Φ([ln(EFW) − ln(median_GA)] / 0.12), where Φ is the standard normal CDF. A weight below the 10th percentile is SGA; above the 90th is LGA. Ultrasound EFW has ±10–15% error, so trends matter more than single values.
Formula
How this is calculated
Fetal weight at any gestational age follows a log-normal distribution: the natural logarithm of weight is approximately normally distributed with a mean equal to ln(median weight at that GA) and a coefficient of variation of about 12% on the log scale, as reported by Hadlock FP et al. (Radiology 1991;181:129–133). This is the most widely used reference for ultrasound-based fetal biometry in clinical practice. The calculator linearly interpolates the median between whole-week values.
The z-score = [ln(EFW) − ln(median)] / 0.12 quantifies how many standard deviations the measured weight lies above or below the median for gestational age. Converting through the standard normal CDF (Φ) gives the percentile. The 10th percentile is the conventional threshold for small for gestational age (SGA) and the 90th for large for gestational age (LGA).
Clinical note: estimated fetal weight from ultrasound biometry carries its own measurement error (typically ±10–15% for individual scans). Percentile classification should always be interpreted in the context of serial growth, amniotic fluid volume, Doppler findings, and maternal risk factors by a qualified obstetric team.
Frequently asked questions
SGA (small for gestational age) means below the 10th percentile for the gestational age; AGA (appropriate) is between the 10th and 90th; LGA (large) is above the 90th. SGA fetuses have a higher risk of poor perinatal outcomes and may need increased monitoring; LGA babies are associated with gestational diabetes and delivery complications.
A fetus grows substantially each week — a 1,700 g baby at 32 weeks is perfectly normal (50th percentile), whereas the same weight at 36 weeks is small (below 10th percentile). Percentile ranking adjusts for gestational age and is therefore a more clinically meaningful measure of growth adequacy.
Ultrasound EFW derived from Hadlock biometric formulas (HC, AC, FL, BPD) typically has a systematic error of ±10–15% for individual scans, rising to ±20% in some cases. Serial measurements over several weeks are more informative than a single scan for assessing growth trajectory.
TG we-Calculate Editorial Team. (2026). Fetal Weight Percentile Calculator — Hadlock 1991 Standards [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fetal-weight-percentile-calculator
TG we-Calculate Editorial Team. "Fetal Weight Percentile Calculator — Hadlock 1991 Standards." TG we-Calculate. 2026. https://we-calculate.com/calculator/fetal-weight-percentile-calculator.
TG we-Calculate Editorial Team, "Fetal Weight Percentile Calculator — Hadlock 1991 Standards," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fetal-weight-percentile-calculator
@misc{wecalculate_fetal_weight_percentile_calculator, title = {Fetal Weight Percentile Calculator — Hadlock 1991 Standards}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fetal-weight-percentile-calculator}}, year = {2026}, note = {TG we-Calculate} }
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