Fetal Weight Percentile Calculator (ultrasound EFW, Hadlock 1991)

Understanding the fetal weight percentile calculator — concept and principle

This calculator places an ultrasound estimated fetal weight (EFW) on the Hadlock 1991 in-utero growth standard and shows its percentile for the week of pregnancy. Parents use it to understand a scan report, and clinicians use percentiles to look for babies who are small or large for gestational age.

EFW vs Hadlock median: with the default inputs the result is 38th percentile (between 10th and 90th).

How it is calculated

Median weight: ln(weight in g) = 0.578 + 0.332 × t − 0.00354 × t², where t is the gestational age in weeks (days ÷ 7 added).

z = (EFW ÷ median − 1) ÷ 0.127

Percentile = share of the normal distribution below z

10th and 90th percentile lines = median × (1 ∓ 1.2816 × 0.127)

The principle

Hadlock and colleagues measured fetuses by ultrasound and kept those later born healthy, so the curve describes normal growth inside the womb. They found that the spread of weights is about the same in relative terms at every age: one standard deviation is 12.7% of the median. That lets a simple z-score be calculated at any week. The z-score is then converted to a percentile, the share of healthy fetuses expected to weigh less.

Worked example

Default inputs: 30 weeks 0 days, EFW 1,500 g.

ln(median) = 0.578 + 0.332 × 30 − 0.00354 × 900 = 7.352 → median 1,559 g.
z = (1,500 ÷ 1,559.3 − 1) ÷ 0.127 = −0.3; weight is −3.8% compared with the median.
Percentile = 38.2 → Between the 10th and 90th.
10th line = 1,306 g, 90th line = 1,813 g.

Each point is the calculator re-run with only “Estimated fetal weight from the scan (g)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Estimated fetal weight from the scan (g)	Percentile for gestational age
375	0
750	0
1,125	1.4
1,500	38.2
1,875	94.5
2,250	100
3,000	100

How to read the result

Where does your result fall?

Below the 10th percentile is usually called small for gestational age, and above the 90th large for gestational age. Many small babies are simply constitutionally small and healthy; doctors look at growth over time, fluid volume and Doppler studies before drawing conclusions. A single estimate also carries an ultrasound error of roughly ±10–15%.

Common mistakes

Using the wrong gestational age. Use the dating from an early scan if you had one.
Comparing percentiles from reports that use different charts, such as INTERGROWTH-21st or WHO fetal charts.
Treating a small change between two scans as real growth when it may be measurement error.

Keep in mind: This is not a diagnosis. Fetal growth is assessed by your obstetric team using repeated scans and other findings.

Frequently asked questions

What percentile is normal for fetal weight?

Between the 10th and 90th percentile is the usual range, but percentiles outside it are not automatically a problem.

Why does my report show a different percentile?

The unit may use a different growth standard or round differently. Ask which chart was used.

How accurate is estimated fetal weight?

Typically within about 10–15% of the birth weight, sometimes more.

Sources: Hadlock FP, Harrist RB, Martinez-Poyer J, "In utero analysis of fetal growth: a sonographic weight standard", Radiology 1991;181(1):129–133

Checked 9 Nov 2026. Hadlock FP, Harrist RB, Martinez-Poyer J, “In utero analysis of fetal growth: a sonographic weight standard”, Radiology 1991;181(1) (regression of ln weight on menstrual age, 1 SD = 12.7% of the predicted weight, Table 1 percentiles).

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