Fetal Percentile Calculator
Turn your ultrasound measurements into your baby’s growth percentile — see exactly where their estimated weight sits on the growth curve, from the 3rd to the 97th, using the same Hadlock standard your sonographer uses.
A percentile simply shows how your baby’s size compares to others at the same week — it is not a grade or a health score. Healthy babies come in every size, from the 5th to the 95th. One measurement is only a snapshot; your sonographer sees the full picture.
Use the gestational age from your ultrasound report (valid for about 14–42 weeks).
You’ll find the EFW on your ultrasound report, usually in grams.
Enter the four measurements from your scan (in millimetres). We’ll calculate the EFW using the Hadlock formula, then find the percentile.
Enter your baby’s gestational age and size to see their growth percentile and where they sit on the curve.
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Where your baby sits on the growth curve
The shaded band spans the 10th to 90th percentile. The lines are the 3rd, 50th, and 97th. Your baby is the marked point.
Your baby on the percentile scale
What this means & what helps
This is an educational estimate, not a diagnosis. Ultrasound weight estimates carry a margin of error of roughly 10–15%, and a single percentile is only a snapshot — your sonographer assesses growth over time alongside amniotic fluid, Doppler flow, and your health. A high or low percentile doesn’t by itself mean anything is wrong. Always follow the percentile and guidance on your official scan report and from your provider.
How the fetal percentile calculator works
Your ultrasound report is full of numbers, and one of the most talked-about is your baby’s growth percentile. This fetal percentile calculator makes that number make sense. It takes your baby’s estimated fetal weight (EFW) and gestational age, compares them against the Hadlock growth standard used in clinics worldwide, and tells you the percentile — for example, the 45th, meaning your baby is estimated to weigh more than 45 out of 100 babies at the same week. If you don’t have the EFW to hand, you can enter the four core measurements instead and the tool will calculate the weight for you.
You can use it two ways. If your report already lists an estimated fetal weight, enter it directly in grams (or pounds and ounces). If it only lists the individual biometry — biparietal diameter, head circumference, abdominal circumference, and femur length — pop those in and the calculator applies the Hadlock four-parameter formula to work out the EFW first. Either way, you get the same result: your baby’s fetal growth percentile and a clear picture of where they land on the curve. If you still need to confirm your dates first, our Crown-Rump Length Calculator can help.
Quick answer
A fetal weight percentile ranks your baby’s estimated weight against others at the same gestational age. Below the 10th percentile is called small for gestational age (SGA), between the 10th and 90th is the normal range (AGA), and above the 90th percentile is large for gestational age (LGA). Most healthy babies fall between the 10th and 90th, and a single number outside that range doesn’t automatically mean there’s a problem.
What is a fetal growth percentile?
A percentile is a way of comparing one baby to many. If your baby is in the 60th percentile for weight, they’re estimated to be heavier than 60% of babies at that exact week of pregnancy, and lighter than 40%. It says nothing about whether your baby is “good” or “behind” — it’s simply a position on a distribution. Just as adults come in a healthy range of sizes, so do babies, and the vast majority sit comfortably somewhere between the 10th and 90th percentiles. Percentiles become useful when they’re tracked over time or when they sit at the far ends of the scale, which is when your provider may look more closely.
SGA, AGA and LGA explained
Your scan report may use one of three shorthand labels for growth, and this baby percentile calculator uses the same ones:
| Label | Percentile | What it means | Typical next step |
|---|---|---|---|
| SGA | Below 10th | Small for gestational age | May prompt growth monitoring & Dopplers |
| AGA | 10th–90th | Appropriate (normal) for gestational age | Routine care |
| LGA | Above 90th | Large for gestational age | May prompt glucose check & delivery planning |
It’s worth repeating: SGA and LGA are descriptions of size, not diagnoses. Many small babies are simply constitutionally small (think petite parents), and many large babies are perfectly healthy. The labels flag babies who may benefit from a closer look, nothing more.
The measurements behind the estimate: BPD, HC, AC, FL
Because a baby can’t be weighed in the womb, sonographers estimate weight from four measurements, and this EFW percentile calculator uses the same ones:
- BPD (biparietal diameter): the width of the head, measured across the top from side to side.
- HC (head circumference): the distance all the way around the head.
- AC (abdominal circumference): the distance around the belly — the single strongest predictor of weight, because it reflects the baby’s liver and fat stores.
- FL (femur length): the length of the thigh bone, a good marker of skeletal growth.
The Hadlock four-parameter formula combines all four for the most reliable estimate. If one measurement is hard to capture — because of the baby’s position or low fluid — the estimate can be less precise, which is one reason a single scan is always read in context.
Fetal weight percentile chart by week
For a quick reference, here are the approximate median (50th percentile) estimated fetal weights by week, based on the Hadlock standard. Your calculator compares your baby to these and everything in between.
| Week | Median EFW | Week | Median EFW |
|---|---|---|---|
| 20 weeks | ~331 g (0.7 lb) | 32 weeks | ~1953 g (4.3 lb) |
| 24 weeks | ~670 g (1.5 lb) | 34 weeks | ~2377 g (5.2 lb) |
| 28 weeks | ~1210 g (2.7 lb) | 36 weeks | ~2813 g (6.2 lb) |
| 30 weeks | ~1559 g (3.4 lb) | 40 weeks | ~3619 g (8.0 lb) |
What affects your baby’s percentile?
Plenty of things shape fetal size, and most are entirely normal. Genetics is the biggest — tall, larger parents tend to have larger babies, and vice versa. Your own health plays a role too: conditions like gestational diabetes can push a baby toward the higher percentiles (LGA), while placental issues or high blood pressure can restrict growth (SGA). Accurate dating matters enormously, because a percentile is only as good as the gestational age it’s compared against — using last-menstrual-period dating instead of an early ultrasound can skew the result. And finally, the inherent margin of error in ultrasound weight estimates (around 10–15%) means the percentile is a well-informed estimate, not a precise measurement.
Should you worry about a high or low percentile?
Usually not on its own. A single percentile at the higher or lower end is a prompt for a closer look, not a verdict. What your provider cares about most is the trend — whether your baby is growing steadily along their own curve — plus supporting checks like amniotic fluid and blood flow. A baby tracking happily along the 8th percentile may be perfectly healthy and simply small, while a baby who drops sharply across percentiles might warrant more attention. If your result here surprises or worries you, that’s exactly the kind of thing to raise at your next appointment.
Related maternal health calculators
This tool is part of the Maternal Health Calculators hub. These companion tools cover dating, growth, and pregnancy monitoring — each answers a different question, so choose whichever fits where you are.
Frequently asked questions
It ranks your baby’s estimated weight against others at the same gestational age. A 60th percentile means your baby is estimated to weigh more than 60% of babies at that week. It’s a position on a distribution, not a grade or a health score.
The calculator estimates fetal weight from ultrasound measurements using the Hadlock formula, then compares it to the median weight for that week using a lognormal distribution. That gives the percentile and the SGA, AGA, or LGA classification.
Anything between the 10th and 90th percentile is considered a normal, appropriate size for gestational age (AGA). Below the 10th is small for gestational age (SGA) and above the 90th is large (LGA) — both are prompts for a closer look, not diagnoses.
Not on its own. Many small babies are simply constitutionally small, especially if the parents are petite. What matters most is the growth trend over time and supporting checks like blood flow and fluid. Your provider interprets a low percentile in that fuller context.
Estimated fetal weight typically carries a margin of error of about 10–15%, so it’s a well-informed estimate rather than an exact figure. Accuracy depends on the scan quality, the baby’s position, and the measurements captured, which is why serial scans are more meaningful than any single one.
Biometry refers to the individual measurements — BPD, HC, AC, and FL. EFW (estimated fetal weight) is the single weight figure calculated from those measurements using a formula like Hadlock. This calculator accepts either, since some reports show only one or the other.
It uses the Hadlock four-parameter formula (BPD, HC, AC, FL) for estimated weight and a Hadlock-based reference for percentiles — the most widely used standard in clinical practice. Other standards such as INTERGROWTH-21st can give slightly different centiles.
You can estimate each twin’s percentile separately using singleton charts, but twins often grow differently from singletons, especially later in pregnancy. Your provider uses twin-specific assessment and monitors the difference between twins, so twin results here are best treated as rough context only.
No. It offers general, educational context for your ultrasound numbers. Your sonographer and provider assess growth over time alongside many other factors and can act on any concern. Always follow your official scan report and your provider’s guidance.
Methodology
This calculator estimates fetal weight and percentile using the Hadlock method. When individual biometry is entered, estimated fetal weight is calculated with the Hadlock four-parameter equation (log10 EFW from BPD, HC, AC, and FL, with measurements converted from millimetres to centimetres). The percentile is then derived by comparing the EFW to the median weight for the gestational age using a lognormal distribution with a log standard deviation of approximately 0.128, tuned so that the modelled 10th and 90th percentiles align with the published Hadlock reference bands. Classification follows standard cut-offs: below the 10th percentile is small for gestational age (SGA), the 10th to 90th percentile is appropriate for gestational age (AGA), and above the 90th percentile is large for gestational age (LGA). Median weights by week are taken from the widely used Hadlock reference. The tool applies over roughly 14 to 42 weeks. Estimated fetal weight carries an inherent margin of error of about 10–15%, and a single percentile is a snapshot rather than a diagnosis. Results are educational and are not a substitute for the assessment on your ultrasound report or care from a qualified provider.
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References & trusted sources
- Hadlock FP et al. (1991) — In utero analysis of fetal growth: a sonographic weight standard (Radiology)
- Perinatology.com — Estimation of Fetal Weight and Age
- ACOG — Fetal Growth Restriction (Practice Bulletin)
- NHS — Ultrasound scans in pregnancy
Last reviewed & updated: July 2026.