EFW Percentile Calculator: Fetal Weight Explained

This calculator estimates your baby’s estimated fetal weight (EFW) percentile based on gestational age and weight. Enter the gestational age in weeks and the estimated fetal weight in grams (from an ultrasound), and you’ll get the percentile rank—showing how your baby’s size compares to a reference population of fetuses at the same age.

Formula

The EFW percentile is derived from a log-normal distribution of fetal weights at each gestational age. The formula used here is based on the Hadlock growth standard, which models fetal weight as a function of gestational age. For a given gestational age, the mean and standard deviation (SD) of the log-transformed weight are:

• Mean log weight (μ) = ln(mean weight at that gestational age)
• SD of log weight (σ) = 0.14 (a constant derived from population data)

The percentile is then calculated using the cumulative distribution function (CDF) of the normal distribution:

Percentile = 100 × Φ( (ln(EFW) – μ) / σ )

Where Φ is the standard normal CDF, EFW is in grams, μ is the natural log of the mean weight for that gestational age (from the Hadlock table), and σ = 0.14. In the calculator, we use the mean weight at 32 weeks (1914.8 g) as a default; for other gestational ages, you would need to look up the corresponding mean.

Worked Example

Using the default values: gestational age = 32 weeks, EFW = 1800 g.

1. Find the mean weight at 32 weeks from the Hadlock table: 1914.8 g.
2. Compute the log of EFW: ln(1800) ≈ 7.4955
3. Compute the log of the mean: ln(1914.8) ≈ 7.5579
4. Difference: 7.4955 – 7.5579 = -0.0624
5. Divide by σ (0.14): -0.0624 / 0.14 = -0.4457
6. Apply the standard normal CDF: Φ(-0.4457) ≈ 0.328
7. Multiply by 100: 32.8%

The calculator shows 32.8%, meaning this baby is smaller than about 33% of fetuses at 32 weeks.

How to Use This Calculator

You’ll need two numbers: your baby’s gestational age (in completed weeks) and the estimated fetal weight (in grams) from an ultrasound report. Most ultrasound reports list EFW in grams; if you have it in pounds and ounces, convert first (1 lb = 453.592 g).

For the most accurate percentile, use the Hadlock reference table for the mean weight at your exact gestational age. The calculator uses a fixed mean for 32 weeks, so for other ages, you should replace the mean in the formula with the appropriate value from a growth chart. Many online resources provide these tables.

Understanding Your Result

The percentile tells you where your baby’s weight falls relative to other fetuses of the same gestational age. For example:

• 10th percentile: 10% of fetuses are smaller, 90% are larger.
• 50th percentile: median weight.
• 90th percentile: 90% of fetuses are smaller.

In clinical practice, EFW below the 10th percentile is often labeled small for gestational age (SGA), while above the 90th percentile is large for gestational age (LGA). However, a single percentile is not a diagnosis—it’s a screening tool. Many healthy babies fall outside these ranges.

Limitations and Common Mistakes

EFW itself has an inherent error of ±15% even with the best ultrasound measurements. This means a baby estimated at 3000 g could actually weigh between 2550 g and 3450 g. The percentile inherits this uncertainty, so don’t overinterpret a single value.

Common mistakes include:

• Using the wrong gestational age (e.g., based on LMP instead of ultrasound dating).
• Entering weight in pounds instead of grams.
• Using a formula meant for a different population (e.g., Hadlock vs. INTERGROWTH-21st).

Always use the same growth standard that your ultrasound report references. Different standards can shift the percentile by 10-20 points.

Why Percentiles Matter

Percentiles help clinicians track growth over time. A single low percentile might just reflect a small but healthy baby, but a drop from the 50th to the 5th percentile between two scans could indicate growth restriction. Conversely, a jump above the 95th percentile may signal macrosomia, which can affect delivery planning.

These percentiles are also used in research to classify outcomes and in counseling parents about expected birth weight. However, they are not predictive of future health or development.

Frequently Asked Questions

What is the difference between EFW percentile and birth weight percentile?

EFW percentile is based on ultrasound measurements during pregnancy, while birth weight percentile is based on actual weight after birth. The same reference charts are often used, but birth weight is exact, whereas EFW is an estimate with error.

Can I use this calculator for any gestational age?

Yes, but you need to update the mean weight in the formula to match your gestational age. The calculator’s default mean is for 32 weeks. For other ages, look up the Hadlock mean weight and replace 1914.8 in the expression.

What if my baby’s EFW is below the 10th percentile?

This is called small for gestational age. It may be normal, but your doctor may recommend additional monitoring, such as Doppler ultrasound or more frequent growth scans, to ensure the baby is getting enough nutrients.

Is the Hadlock standard the only one?

No. Other standards include INTERGROWTH-21st, WHO, and local population charts. They can give different percentiles because they are based on different populations. Always use the same standard as your ultrasound report.

How accurate is the EFW percentile?

The percentile is only as accurate as the EFW measurement, which has a margin of error of about ±15%. So a percentile of 30% could realistically be anywhere from 15% to 50%. It’s a guide, not a precise number.

Can I calculate EFW percentile from abdominal circumference and femur length?

Not directly. Those measurements are used to estimate fetal weight first (using formulas like Hadlock), and then the EFW is compared to a growth chart. This calculator takes the EFW as input, so you’d need to compute that separately.

Related

• Average Life Expectancy Calculator: How It Works
• Average Pace Calculator: Formula & How to Use
• CAC Percentile Calculator: What It Is & How to Use

in ga | Gestational age | weeks | 32 | 0
in efw | Estimated fetal weight | grams | 1800 | 0
out percentile | EFW percentile | % | 100 * 0.5 * (1 + erf((ln(efw/1914.8)) / (0.14 * sqrt(2)))) | 1
out zscore | Z-score | - | (ln(efw/1914.8)) / 0.14 | 2