How Fetal Weight Percentile Is Calculated
Fetal weight percentile compares an estimated fetal weight (EFW) — usually generated by an ultrasound machine from measurements like head circumference, abdominal circumference, and femur length — against a reference population of fetuses at the same gestational age. This calculator takes an EFW you already have (from an ultrasound report) plus gestational age, and estimates where that weight falls on a Hadlock-style growth curve. This is an educational approximation, not a substitute for the official percentile printed on your ultrasound report or your clinician's interpretation.
The calculation method
The tool looks up the median (50th percentile) EFW for your exact gestational age from a Hadlock-style reference curve (interpolated between weekly landmark values from 14 to 42 weeks), then models fetal weight at that gestational age as approximately normally distributed with a standard deviation of about 12% of the median — a coefficient of variation commonly cited for third-trimester EFW. From there it computes a z-score, z = (EFW − median) ÷ (0.12 × median), and converts that z-score into a percentile using the standard normal distribution. A z of 0 gives the 50th percentile; positive z-scores push the percentile above 50, negative z-scores push it below.
Understanding SGA, AGA, and LGA
Clinically, an EFW below the 10th percentile for gestational age is classified as small for gestational age (SGA), between the 10th and 90th percentile is appropriate for gestational age (AGA), and above the 90th percentile is large for gestational age (LGA). SGA can sometimes signal fetal growth restriction and typically prompts closer monitoring — repeat growth scans, amniotic fluid assessment, and umbilical artery Doppler studies. LGA can raise considerations around gestational diabetes screening and delivery planning. Neither label alone is a diagnosis; trend over multiple scans and the full clinical picture matter more than one number.
Limitations of this estimate
Real clinical fetal growth charts (Hadlock 1991, INTERGROWTH-21st, WHO, Fenton for preterm infants) use full published percentile tables rather than a single fixed 12% coefficient of variation, and different charts can produce slightly different percentiles for the same EFW. Ultrasound-based EFW itself typically carries a margin of error of roughly 8-15%. Use this calculator to build intuition about how a given weight compares to typical growth for that gestational age, but treat the percentile on your official ultrasound report — interpreted by your obstetric provider — as the number that matters clinically.