EFW Percentile Calculator by Gestational Age
The calculator for this page is switched off while it is checked. The formula is below, and the worked example in the article shows how to use it by hand.
This calculator estimates the percentile rank of an estimated fetal weight (EFW) for a given gestational age, based on standard fetal growth reference data. You enter the gestational age in weeks and the estimated fetal weight in grams, and the calculator returns the percentile, helping you understand how that weight compares to a reference population of fetuses at the same age.
Formula
The percentile is derived from a log-normal distribution of fetal weights at each gestational age. The formula used is:
Percentile = 100 × Φ( (ln(EFW) – μ) / σ )
Where:
- EFW = estimated fetal weight (grams)
- μ = mean of the natural logarithm of fetal weight at the given gestational age (grams, log scale)
- σ = standard deviation of the natural logarithm of fetal weight (dimensionless)
- Φ = cumulative distribution function of the standard normal distribution
In the calculator, μ and σ are approximated using a polynomial fit to reference data (e.g., Hadlock growth curves). For the default gestational age of 32 weeks, the mean log weight is approximately 7.5 (corresponding to ~1800 g) and σ is about 0.1.
Worked Example
Using the default values: gestational age = 32 weeks, EFW = 1800 grams.
- Calculate μ: μ = 0.0000000001×32^6 – 0.00000001×32^5 + 0.0000004×32^4 – 0.00001×32^3 + 0.0002×32^2 – 0.001×32 + 0.05 ≈ 7.5
- Calculate ln(EFW) = ln(1800) ≈ 7.4955
- Compute z-score: (7.4955 – 7.5) / 0.1 = -0.045
- Find Φ(-0.045) ≈ 0.482
- Percentile = 100 × 0.482 = 48.2%
The calculator rounds to 48.2%, meaning this EFW is at the 48th percentile for 32 weeks.
Understanding Fetal Weight Percentiles
Fetal weight percentiles are used by obstetricians to assess growth relative to a reference population. A percentile of 50 means the fetus is exactly average. Values between 10 and 90 are generally considered normal. Below 10th percentile may indicate small for gestational age (SGA), while above 90th may indicate large for gestational age (LGA). However, a single measurement is not diagnostic; trends over time matter more.
How to Choose a Reference Population
Different growth charts exist, such as Hadlock, INTERGROWTH-21st, and WHO. Each is based on different populations and ultrasound methods. The calculator uses a generic polynomial approximation to Hadlock-style data, which is common in many clinical settings. For specific populations (e.g., high-altitude, multiple gestations), the percentile may differ. Always discuss with your healthcare provider which chart they use.
Common Mistakes and Limitations
- Unit mix-ups: Ensure weight is in grams, not kilograms or pounds. 1 kg = 1000 g; 1 lb ≈ 453.6 g.
- Gestational age: Use completed weeks (e.g., 32 weeks, not 32.5). Some charts use exact days.
- EFW accuracy: Ultrasound EFW has a margin of error of ±10-15%, so the percentile is an estimate, not an exact value.
- Single measurement: A single low percentile does not necessarily indicate a problem; serial measurements are needed.
Interpreting the Result
The result is a percentage between 0 and 100. For example, a percentile of 25 means the fetus is heavier than 25% of fetuses at that gestational age. A percentile of 50 is median. A percentile below 10 or above 90 warrants further evaluation, such as Doppler studies or additional ultrasounds. Remember that the normal range is wide; many healthy babies are at the extremes.
Alternatives and Related Tools
Other calculators estimate EFW from biometric measurements (e.g., BPD, HC, AC, FL) using formulas like Hadlock. This calculator assumes you already have an EFW. If you have raw measurements, use a dedicated EFW calculator first. Also, some charts use abdominal circumference alone for growth assessment. Always combine percentile with other clinical indicators.
Frequently Asked Questions
What is a normal EFW percentile?
Typically, the 10th to 90th percentile is considered normal. However, some fetuses are constitutionally small or large. Your doctor will look at your overall health, genetics, and growth trajectory.
Can I use this calculator for twins?
No, this calculator is based on singleton pregnancies. Twin growth patterns differ significantly, and dedicated twin charts should be used.
Why does my doctor use a different percentile?
Different clinics may use different reference charts (e.g., Hadlock vs. INTERGROWTH). This can lead to slight variations in percentile. Always confirm which chart your provider uses.
How accurate is the ultrasound EFW?
Ultrasound EFW has an error of about ±10-15%. Therefore, the percentile is an approximation. A change of a few percentile points between scans is usually not significant.
What if my baby is below the 10th percentile?
It may indicate growth restriction, but many factors affect weight. Your doctor may recommend more frequent ultrasounds, Doppler studies, or other tests to ensure the baby is healthy.
Related
Formula used
in w | Gestational age | weeks | 32 | 0 in g | Estimated fetal weight | grams | 1800 | 0 out p | EFW percentile | % | 100 * 0.5 * (1 + erf((ln(g / (0.0000000001 * w^6 - 0.00000001 * w^5 + 0.0000004 * w^4 - 0.00001 * w^3 + 0.0002 * w^2 - 0.001 * w + 0.05)) / (0.1 * sqrt(2)))) | 1
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