The formula
How to calculate fetal growth percentile
Fetal growth percentiles place an estimated weight against the range expected at that gestational age. They are how ultrasound reports flag babies who may be growing too slowly or unusually large.
The median curve here is the Hadlock in-utero growth reference, with variability set at roughly 12% of the median. The percentile is derived from a normal approximation, which is close to standard charts through the third trimester.
What to enter:
- Gestational age (weeks)
- Estimated fetal weight (g) — from the ultrasound report, usually labelled EFW
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
None of the fields above need specialist equipment to fill in — the inputs are the kind of measurements that can be taken at home with a tape measure or scale, which is exactly why this style of calculation is widely used as a first, approximate check rather than a diagnostic one.
Why fetal growth percentile matters
People usually look up a fetal growth percentile calculation to put a number on something they can already sense but cannot quite quantify — how their fitness compares with a reference range, how a measurement is trending, or what a change actually means. A calculator turns that vague sense into a concrete figure, which is useful even though the figure itself is only ever an estimate.
It is most useful tracked over time rather than read once — a single reading tells you where you are today, while the same calculation repeated weeks or months apart, using the same method each time, shows the direction things are actually moving in.
None of the figures produced on this page are intended to replace a proper assessment, and nothing here should be read as a diagnosis or a treatment recommendation. What a calculator like this is good for is preparation — arriving at an appointment with an actual number already in hand, rather than a vague sense of a change, tends to make that conversation more useful for everyone involved.
A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.
Worked example
Here is the calculation with the starting values:
- Gestational age: 32 weeks
- Estimated fetal weight: 1,900 g
That gives:
- Approximate percentile: 40.52 %
- Median weight at this gestation: 1,952.84 g
- Difference from the median: -52.84 g
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Below the 10th percentile is described as small for gestational age, and above the 90th as large for gestational age. Neither is a diagnosis on its own — the trend across successive scans matters far more than a single reading.
Where this goes wrong. Estimated fetal weight from ultrasound carries an error of roughly 10–15% in itself, which is comparable to the difference between adjacent percentile bands. UK maternity care increasingly uses customised charts adjusted for maternal height, weight, ethnicity and parity. Always rely on your clinical team rather than a calculator.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Typically within 10–15% of the actual birth weight, which at 32 weeks is a range of several hundred grams. Accuracy is worst at the extremes of size, which is precisely where it matters most.
It may mean a constitutionally small but perfectly healthy baby, or it may indicate restricted growth needing monitoring. Serial scans showing the trend, together with Doppler studies, are what distinguish the two.
The answer it gives you is approximate percentile. With 32 weeks gestational age and 1,900 g estimated fetal weight, that comes to 40.52 %. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.