What Is a Fetal Weight Percentile?
Fetal weight percentile tells you how your baby's estimated weight compares to other babies at the same gestational age. If your baby is at the 50th percentile, it means 50% of babies at that gestational age weigh less, and 50% weigh more. Percentiles help doctors detect significant growth abnormalities without relying on a single fixed "normal" weight.
SGA and LGA Definitions
Two key clinical thresholds matter most:
- SGA – Small for Gestational Age: babies below the 10th percentile. More than 90% of the population grew larger. SGA may indicate intrauterine growth restriction (IUGR) and warrants closer monitoring.
- LGA – Large for Gestational Age: babies above the 90th percentile. Only 10% of the population is larger. LGA can be associated with gestational diabetes or macrosomia.
- AGA – Appropriate for Gestational Age: babies between the 10th and 90th percentile. This is the normal range.
How to Use This Calculator
- Select your measurement system: Metric (mm) or Imperial (inches).
- Enter gestational age in weeks (14–42 weeks).
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Enter the four ultrasound measurements from your sonography report:
- BPD – Biparietal Diameter (width of the baby's skull)
- HC – Head Circumference
- AC – Abdominal Circumference (measured at liver level)
- FL – Femur Length (thigh bone)
- Click Calculate to see the estimated fetal weight and percentile.
The Hadlock Formula
This calculator uses the Hadlock (1985) formula — the most widely validated equation for estimating fetal weight from ultrasound biometry:
log₁₀(EFW) = 1.3596 − 0.00386×AC×FL + 0.0064×HC + 0.00061×BPD×AC + 0.0424×AC + 0.174×FL
All measurements are converted to centimeters internally. The result (EFW) is in grams. Typical accuracy: ±15% of actual birth weight.
Fetal Growth Chart by Week
The table below shows INTERGROWTH-21st reference values (mean ± SD):
| GA (weeks) | 3rd pct (g) | 10th pct (g) | 50th pct (g) | 90th pct (g) | 97th pct (g) |
|---|---|---|---|---|---|
| 20 | 248 | 280 | 330 | 388 | 437 |
| 22 | 355 | 401 | 475 | 560 | 631 |
| 24 | 493 | 558 | 663 | 782 | 883 |
| 26 | 666 | 755 | 899 | 1062 | 1200 |
| 28 | 876 | 995 | 1183 | 1399 | 1581 |
| 30 | 1122 | 1274 | 1516 | 1793 | 2027 |
| 32 | 1403 | 1595 | 1894 | 2240 | 2533 |
| 34 | 1714 | 1948 | 2308 | 2730 | 3086 |
| 36 | 2040 | 2318 | 2738 | 3235 | 3658 |
| 38 | 2365 | 2689 | 3161 | 3735 | 4218 |
| 40 | 2682 | 3045 | 3553 | 4198 | 4738 |
Frequently Asked Questions
What percentile is normal for a fetus?
Any percentile between the 10th and 90th is considered appropriate for gestational age (AGA). A result at the 50th percentile means average weight for that week of pregnancy.
My baby is at the 5th percentile — should I be worried?
Below the 10th percentile (SGA) indicates the baby is smaller than 90% of peers. This may be constitutional (genetically small parents) or may reflect growth restriction. Your OB will order additional tests such as Doppler blood flow studies to evaluate further.
How accurate is fetal weight estimation by ultrasound?
Ultrasound-based EFW has a typical margin of error of ±10–15% of actual birth weight. The Hadlock formula used here is considered the gold standard and is used worldwide.
What ultrasound measurements do I need?
You need four values from your sonography report: BPD (biparietal diameter), HC (head circumference), AC (abdominal circumference), and FL (femur length). These are standard measurements taken during any anatomy scan.
At what gestational age is this calculator valid?
This calculator is validated for gestational ages 14–42 weeks. Ultrasound biometry is most accurate between weeks 18 and 36.