Homeโ€บCalculatorsโ€บHealthโ€บFetal Weight Percentile Calculator

Fetal Weight Percentile Calculator

Health

Estimate a fetus's weight percentile for gestational age from an ultrasound estimated fetal weight (EFW), compared against a reference growth curve.

Reviewed by the thecalcu.com team ยท Last updated July 7, 2026

2040
2005,000

Fetal Weight Percentile

50
Reference Median EFW for Gestational Age
1,700
z-score
0

This calculator computes your Fetal Weight Percentile, Reference Median EFW for Gestational Age, z-score from the values you enter.

Inputs
Gestational AgeEstimated Fetal Weight (EFW)
Outputs
Fetal Weight PercentileReference Median EFW for Gestational Agez-score

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.

Why Use a Fetal Weight Percentile Calculator?

Ultrasound growth scan reports can be dense with numbers, and this calculator gives a quick, independent way to check how an EFW reading translates into a percentile. It's useful for understanding scan results between appointments or for general reference while researching what's typical at a given stage of pregnancy.

Who Should Use This Calculator?

  • Expecting parents who want to double-check an EFW percentile mentioned in an ultrasound report.
  • Anyone researching typical fetal growth patterns by gestational week.
  • Students studying prenatal growth reference curves.
  • Parents preparing questions for an upcoming prenatal appointment.

What Insights Does the Fetal Weight Percentile Calculator Give You?

The Fetal Weight Percentile is the headline figure, showing where the entered EFW ranks against reference fetuses at the same gestational age. The Reference Median EFW for Gestational Age shows the expected midpoint weight at that stage, and the z-score shows how far the entered EFW is from that median in standard deviation units.

How to use this Fetal Weight Percentile calculator

  1. Enter the gestational age in weeks, as reported on your ultrasound scan.
  2. Enter the estimated fetal weight (EFW) in grams from your scan report.
  3. Read the Fetal Weight Percentile result instantly.
  4. Compare against the Reference Median EFW shown for that gestational age.
  5. Re-run the calculator at each scan to track how the percentile trends across pregnancy.
Show formula & methodology โ†“Show less โ†‘

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 โˆ’ Reference median) รท Reference SD

Percentile = ฮฆ(z-score) ร— 100, where ฮฆ is the cumulative standard normal distribution.

Worked example, an EFW of 1,900 grams at 33 weeks:

Reference median at 33 weeks โ‰ˆ 1,975 g, SD โ‰ˆ 275 g (interpolated between the 32 and 36-week reference points)

z-score = (1,900 โˆ’ 1,975) รท 275 = โˆ’0.27

Percentile โ‰ˆ ฮฆ(โˆ’0.27) 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

What is fetal weight percentile?
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.
What is an estimated fetal weight (EFW)?
EFW is a weight estimate calculated during a prenatal ultrasound from measurements like head circumference, abdominal circumference, and femur length, using standard obstetric formulas, it's an estimate, not a direct weight measurement.
How is fetal weight percentile calculated?
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.
What does it mean if fetal weight percentile is below the 10th percentile?
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.
How accurate is an ultrasound EFW?
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, this is a normal limitation of ultrasound-based estimation, not a sign of error in the scan.
What is the difference between fetal weight percentile and birth weight percentile?
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, the two often align but can differ given the estimation margin of error in ultrasound EFW.
What gestational age range does this calculator support?
This tool covers gestational ages from 20 to 40 weeks, spanning the second and third trimesters when EFW is routinely measured.
Why do doctors track fetal weight percentile across multiple scans?
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.
Can twins or multiples use this calculator accurately?
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.
Is a high fetal weight percentile a concern?
Fetal weight percentiles above the 90th percentile are sometimes evaluated for factors like gestational diabetes, but a high percentile alone is not a diagnosis, it's one data point your care provider considers alongside the full clinical picture.
Also known as
EFW percentile calculatorfetal growth percentileestimated fetal weight calculatorultrasound weight percentile