How the Fetal Weight Percentile Quiz works
This tool estimates fetal weight from standard ultrasound biometry using the Hadlock formula, then places that estimate on an approximate growth curve for the entered gestational age. It is an educational estimate meant to help you understand how sonographic fetal-weight percentiles are derived — it is not a diagnostic tool, and it does not replace the interpretation your obstetric provider or sonographer gives from the actual ultrasound images, growth trend, and your full pregnancy history.
The Hadlock estimated fetal weight (EFW) formula
Enter the four standard biometry measurements from an ultrasound report, each in centimeters: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The calculator applies the widely used Hadlock (1985) composite formula: log10(EFW) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL, where EFW comes out in grams once you raise 10 to that power. This four-measurement version is the formula most ultrasound machines default to because it tends to track birth weight closely across a wide range of gestational ages.
Converting EFW into an approximate percentile
Fetal weight is not evaluated in isolation — the same weight can be perfectly normal at 36 weeks and concerning at 28 weeks. This calculator compares your EFW to the average fetal weight expected at the gestational age you entered, based on the Hadlock (1991) fetal growth reference, and assumes weight is distributed approximately log-normally around that average (roughly a ±10-15% spread, one standard deviation). The result is expressed as an approximate percentile: the 50th percentile means your estimate matches the reference average for that gestational age, while lower or higher percentiles mean the estimate falls below or above that average. Because real clinical growth charts vary slightly (Hadlock, INTERGROWTH-21st, WHO, or a practice's own customized chart) and depend on population and measurement technique, treat the percentile shown here as an approximation for learning purposes, not the exact number your sonographer's machine would report.
Reading SGA, AGA, and LGA
- AGA (appropriate for gestational age): roughly the 10th-90th percentile — the most common range.
- SGA (small for gestational age): below the 10th percentile; below the 3rd percentile is more closely monitored, since it can (but does not always) reflect growth restriction.
- LGA (large for gestational age): above the 90th percentile; above the 97th percentile or an EFW above roughly 4,000-4,500 g raises macrosomia considerations, particularly for delivery planning.
Common pitfalls
- Mixing up units: all four biometry inputs must be in centimeters, not millimeters or inches, or the formula will be off by orders of magnitude.
- Using the wrong gestational age: percentile depends heavily on gestational age — an EFW that is average at 34 weeks could be far above average at 30 weeks.
- Treating one ultrasound as the full picture: a single EFW percentile matters less than the growth trend across multiple scans, and it's normal for estimates to shift somewhat between visits due to measurement variability.