Fetal Weight Percentile Calculator
HealthEstimate a fetus's weight percentile for gestational age from an ultrasound estimated fetal weight (EFW). Compare EFW against a reference growth curve instantly.
Fetal Weight Percentile
50
Reference Median EFW for Gestational Age
1,700
z-score
0
What is a Fetal Weight Percentile?
The Fetal Weight Percentile Calculator estimates where a prenatal ultrasound's estimated fetal weight (EFW) ranks against reference growth curve data for the same gestational age. Ultrasound reports commonly include a percentile alongside the raw EFW figure, and this tool lets you reproduce that comparison using a reference growth curve.
It complements the Birth Weight Percentile Calculator for after delivery and the Pregnancy Due Date Calculator for tracking gestational age.
How to use this Fetal Weight Percentile calculator
- Enter the gestational age in weeks, as reported on your ultrasound scan.
- Enter the estimated fetal weight (EFW) in grams from your scan report.
- Read the Fetal Weight Percentile result instantly.
- Compare against the Reference Median EFW shown for that gestational age.
- Re-run the calculator at each scan to track how the percentile trends across pregnancy.
Formula & Methodology
The calculator interpolates a reference median EFW and standard deviation (SD) for the entered gestational age from a reference growth curve, then computes: z-score = (EFW โ Reference median) รท Reference SD Percentile = ฮฆ(z-score) ร 100, where ฮฆ is the cumulative standard normal distribution. Worked example โ an EFW of 1,900 grams at 33 weeks: Reference median at 33 weeks โ 1,975 g, SD โ 275 g (interpolated between the 32 and 36-week reference points) z-score = (1,900 โ 1,975) รท 275 = โ0.27 Percentile โ ฮฆ(โ0.27) ร 100 = 39th percentile This shows the estimated fetal weight is close to, and slightly below, the reference median for 33 weeks.
Frequently Asked Questions
Fetal weight percentile compares an ultrasound's estimated fetal weight (EFW) against reference growth curve data for the same gestational age, showing what percentage of fetuses at that stage of pregnancy are estimated to weigh less. It's a standard part of how ultrasound growth measurements are reported.
EFW is a weight estimate calculated during a prenatal ultrasound from measurements like head circumference, abdominal circumference, and femur length, using standard obstetric formulas โ it's an estimate, not a direct weight measurement.
The calculator compares the entered EFW to a reference median weight for the same gestational age in weeks, computes a z-score using reference standard deviation data, and converts that z-score to a percentile using the normal distribution.
A percentile below 10 is one criterion clinicians consider when evaluating for fetal growth restriction, though a single ultrasound estimate has a margin of error and is typically interpreted alongside other measurements and trends over multiple scans.
Ultrasound EFW typically carries a margin of error of around 10-15%, meaning the actual birth weight can reasonably fall meaningfully above or below the estimate โ this is a normal limitation of ultrasound-based estimation, not a sign of error in the scan.
This calculator uses a prenatal ultrasound estimate before birth, while the [Birth Weight Percentile Calculator](/birth-weight-percentile-calculator/) uses the actual weight measured at delivery โ the two often align but can differ given the estimation margin of error in ultrasound EFW.
This tool covers gestational ages from 20 to 40 weeks, spanning the second and third trimesters when EFW is routinely measured.
A single percentile reading is a snapshot, but tracking the trend across two or more scans shows whether growth is following a consistent curve, which is often more informative than any one measurement in isolation.
This calculator's reference data reflects singleton pregnancies; twins and multiples typically follow a somewhat different growth trajectory in the third trimester, so results for multiples should be interpreted with that context.
Fetal weight percentiles above the 90th percentile are sometimes evaluated for factors like gestational diabetes, but a high percentile alone is not a diagnosis โ it's one data point your care provider considers alongside the full clinical picture.
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