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Fetal Weight Percentile Calculator

Find the weight percentile and the median weight expected for the gestational age.

How it is calculated

The tool first estimates the median weight for the week, then places your figure on a normal curve around it:

median = exp(0.578 + 0.332·GA − 0.00354·GA²) grams, with GA in weeks

z = (weight − median) / (0.127 × median)

The standard deviation is fixed at 12.7 % of the median, the uniform spread (1 SD) reported by Hadlock et al. (1991). The z-score is turned into a percentile with a normal-distribution approximation, then limited to the range 0.1 to 99.9. Gestational age must be 10 to 42 weeks and weight in grams above zero. This is a single built-in Hadlock-type curve, not the INTERGROWTH-21st or Fenton charts, and it ignores sex and ethnicity, so a chart used by your clinic can give a different percentile. Use it for orientation, not diagnosis.

Worked example

A scan at 32 weeks gives an estimated fetal weight of 1800 g.

Median = exp(0.578 + 0.332 × 32 − 0.00354 × 32²) = exp(0.578 + 10.624 − 3.625) = exp(7.577) ≈ 1,953 g.

SD = 0.127 × 1,953 ≈ 248 g, so z = (1800 − 1953) / 248 ≈ −0.62.

That z corresponds to the 26.9th percentile: the baby is a little under the median but well inside the normal range.

Try this example →

Common mistakes

  • Entering the weight in kilograms or pounds. The field is grams, so 1.8 gives a percentile at the very bottom of the scale.
  • Entering the baby's expected birth weight or the mother's weight instead of the estimated fetal weight from the scan report.
  • Using the gestational age counted in months, or the date-of-conception age. The model uses weeks of gestation counted from the last period (or the dating scan).
  • Treating a low percentile as a diagnosis of growth restriction, or a percentile in the 90s as macrosomia. One estimate has a sizeable measurement error, and trends across several scans matter more.
  • Comparing this percentile with the one in the ultrasound report and assuming one is wrong. Reports often use other charts, so the numbers can legitimately differ.

Sources

About this calculator

Estimated fetal weight (EFW), calculated from ultrasound biometry measurements using formulas like Hadlock’s, is compared against the expected weight for that gestational age to produce a percentile. This calculator uses one built-in Hadlock-type median curve with a fixed spread of 12.7% (the one standard deviation Hadlock et al. reported in 1991), the same for every baby, rather than sex- or ethnicity-specific charts such as INTERGROWTH-21st or Fenton, so treat it as an approximation. That percentile flags babies who are small for gestational age (possible growth restriction) or large for gestational age (possible macrosomia).

Obstetricians and sonographers use it during routine prenatal ultrasounds to track a fetus’s growth trajectory across pregnancy, and it becomes especially important in monitoring high-risk pregnancies where growth can shift quickly.

Gestational age and estimated fetal weight go in; the weight percentile comes out along with the median weight expected at that gestational age.

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