Fetal Weight Percentile Calculator
Estimate what percentile an ultrasound estimated fetal weight falls in for its gestational week, using Hadlock reference growth.
How it works
z = (weight − median) ÷ (0.12 × median) · percentile = Φ(z) × 100, using Hadlock 50th-percentile medians and a 12% coefficient of variation.
Estimated fetal weight from ultrasound is compared against a growth reference to give a percentile — the share of babies at that gestational week who would weigh less. This tool uses Hadlock 50th-percentile medians and approximates the spread with a 12% coefficient of variation, so the 10th-to-90th-percentile band covers the typical range. Ultrasound weight itself has an inherent error of about 10–15%, and a single measurement matters less than the growth trend across scans, which your provider interprets.
Worked example
At 28 weeks the median is about 1210 g. An estimated weight of 1200 g gives z = (1200 − 1210) ÷ 145 ≈ −0.07, or roughly the 47th percentile — squarely in the typical range.
Sources: Hadlock fetal growth reference (Wikipedia: Estimated fetal weight)
Frequently asked questions
What percentile is considered normal?
Broadly, the 10th to 90th percentile is the typical range. Below the 10th may prompt a look for growth restriction and above the 90th for a large baby, but a single number is only a starting point — the trend across scans matters more.
How accurate is an ultrasound weight estimate?
Estimated fetal weight is typically within about 10–15% of the true birth weight, so percentiles are approximate. Consistent measurements over several scans are more informative than any one estimate.
My baby measured small or large — should I worry?
Not on one reading alone. Position, fluid and technique all affect the estimate. Your provider looks at the growth pattern, your history and other checks together before drawing conclusions.
Related calculators
Educational estimates from standard formulas and population averages — every pregnancy and baby is different. This is not medical advice; your doctor, midwife or pediatrician, with an examination or ultrasound, gives the accurate guidance.