Fetal Weight Percentile Calculator
Calculate estimated fetal weight percentile from ultrasound measurements using standard growth references.
What Is Fetal Weight Percentile?
Fetal weight percentile compares an estimated fetal weight (EFW) with a reference population at the same gestational age. A result at the 25th percentile means the fetus weighs more than about 25% of fetuses at that gestational age. Use this alongside the Crown-Rump Length Calculator and Baby Percentile Calculator for broader growth assessment.
How EFW Is Calculated
This tool supports Hadlock formulas from ultrasound biometry (BPD, AC, FL in millimeters) or direct entry of an EFW already reported on an ultrasound. Hadlock 4-parameter EFW uses:
$$\log_{10}(EFW) = 1.335 - 0.0034 \times AC \times FL + 0.0316 \times BPD + 0.0457 \times AC + 0.1623 \times FL$$
Percentile is derived from a simplified z-score against gestational-age mean and standard deviation references (weeks 20–42), adjusted for fetal sex.
Growth Categories
- SGA (small for gestational age): below the 10th percentile
- AGA (appropriate for gestational age): 10th to 90th percentile
- LGA (large for gestational age): above the 90th percentile
Frequently Asked Questions
Which ultrasound measurements do I need?
For the full Hadlock formula, enter BPD, abdominal circumference (AC), and femur length (FL) in millimeters. If only AC and FL are available, choose the AC + FL method.
What gestational ages are supported?
Percentile estimation is designed for gestational ages from 20 to 42 weeks. Enter completed weeks plus additional days (0–6).
Does fetal sex change the percentile?
Yes. Male and female fetuses have slightly different reference means, so selecting the correct sex improves percentile accuracy.
When should I worry about a low percentile?
A single reading below the 10th percentile warrants clinical review, especially if growth velocity is slowing or Doppler findings are abnormal. Always interpret with your obstetric team.
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