What this Fetal Weight Percentile Calculator does
After a growth scan your ultrasound report lists an estimated fetal weight (EFW) in grams. This English-language tool takes that number plus the gestational age in weeks and returns the corresponding percentile on the Hadlock growth reference curve — the standard published in Radiology (1991) that most perinatology practices still use. A result at the 50th percentile means the fetus is at the population median for that week; below the 10th is conventionally called small for gestational age (SGA), above the 90th is large for gestational age (LGA). The calculation runs 100% client-side — your data never leaves your browser. No uploads, no tracking, no server logs. If you're also monitoring maternal nutrition, our [BMI calculator](/en/bmi-calculator/) is a useful companion for pre-pregnancy and postpartum context.
Features
- Hadlock reference data. Percentiles are derived from the Hadlock 1991 fetal growth standard, the most widely cited sonographic weight reference in perinatology.
- SGA / AGA / LGA classification. The result is automatically labeled as small, appropriate, or large for gestational age using the conventional 10th and 90th percentile cut-offs.
- Full gestational age range. Accepts EFW inputs from 20 to 42 completed weeks, covering the range where growth scans are routinely ordered.
- Trend-ready output. Run the calculator for each scan visit and note the percentile shift — tracking direction across multiple readings matters more than any single value.
- Instant, private calculation. Everything runs in JavaScript in your browser. No form submission, no server round-trip, no account required — consistent with how privacy-conscious antenatal growth chart tools should work.
- Plain-language result. Output includes the numeric percentile and a short interpretation so you can bring a clear question to your next obstetric appointment.
How to use the Fetal Weight Percentile Calculator
You need two numbers from your ultrasound report: the EFW in grams and the gestational age in completed weeks.
- Find your EFW. Locate the 'Estimated Fetal Weight' line on your scan report. It is usually expressed in grams, e.g.
1850 gat 32 weeks. - Enter gestational age. Type the completed weeks (e.g.
32). Use the week shown on the report, not a rounded due-date estimate. - Enter the EFW. Type the EFW in grams exactly as reported. Do not convert to pounds — the calculator expects grams.
- Click Calculate. The tool returns the percentile, the SGA/AGA/LGA label, and a brief note on what the result means in population terms.
- Compare across visits. Repeat for each scan to build a trend. A fetus tracking at the 30th percentile consistently is generally less concerning than one that drops from the 50th to the 12th between visits.
Common use cases
- Understanding a growth scan result. A Boston-based OB practice may report an EFW without including the percentile. This calculator lets you look it up before your follow-up appointment so you arrive with a specific question rather than general anxiety.
- Tracking fetal growth percentile across scans. Entering the EFW from each visit lets you visualize whether the fetus is maintaining, crossing up, or crossing down percentile lines — the key clinical signal in antenatal growth monitoring.
- Patient education in clinical settings. Midwives and childbirth educators can use the tool on a waiting-room tablet to explain what 'estimated fetal weight percentile' means without transmitting patient data to any third-party server.
- Research and academic reference. Students studying perinatology or working through a fetal growth chart calculator exercise can verify Hadlock-based percentile outputs against published tables without needing specialized software.
Frequently asked questions
Does this tool calculate EFW from biometry measurements like BPD, HC, AC, or FL?
No. This tool takes an EFW you already have from your ultrasound report and converts it to a percentile. It does not derive EFW from raw biometry. For that step you need dedicated perinatology software or your sonographer's workstation.
Is my data sent to a server or stored anywhere?
Nothing leaves your device. The entire calculation runs in your browser using JavaScript. There is no form submission, no analytics on the inputs you type, and no server log that records your EFW or gestational age. This is the same privacy model used by offline clinical reference apps.
My baby is below the 10th percentile — should I be worried?
A single reading below the 10th percentile (conventionally called SGA) is not a diagnosis. Clinicians look at the growth trajectory across multiple scans, amniotic fluid index, Doppler flow studies, and your full obstetric history. One data point without context can be misleading — your managing provider has all of that context and is the right person to interpret the result.
Which growth reference does this calculator use?
It uses the Hadlock 1991 sonographic weight standard (Radiology, 181:129–133), which remains the most widely cited reference in perinatology for estimated fetal weight percentile calculations. Some institutions use alternative curves (Intergrowth-21, Fenton); if your practice uses a different reference, small numerical differences are expected.
How accurate is a percentile estimate from ultrasound EFW?
Ultrasound EFW itself carries an inherent measurement error of roughly ±10–15% in the third trimester. Because the percentile is derived from that estimate, treat the result as a useful approximation, not a precise measurement. The Hadlock curve approximation used here also applies a standard deviation of about 12% of the mean weight — consistent with published data, though the full original table is not fully reproduced. Adolphe Quetelet's early work on population distributions, described in the Adolphe Quetelet (Britannica) article, laid the statistical groundwork that underpins these reference curve methods.
What does 'antenatal growth chart calculator' mean — is it the same thing?
Yes, the terms 'antenatal growth chart calculator', 'fetal growth percentile calculator', and 'perinatology growth percentile calculator' all refer to tools that map an EFW to a reference population for a given gestational week. They differ mainly in which growth standard they use (Hadlock, Intergrowth-21, etc.). This tool uses the Hadlock 1991 curve. For reference on how modern browser APIs handle encoded data securely on the client side, see MDN — encodeURIComponent — the same privacy-first philosophy applies here.