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Estimated Fetal Weight (EFW) Calculation

Calculate estimated fetal weight from biometric values measured on ultrasound using the Hadlock formula

Ultrasound / Fetal Biometry

Medical content owner: Op. Dr. Cem Akaltun, Obstetrician and Gynecologist · English version: AI-assisted translation of the Turkish original, not separately reviewed by a physician · Editorial policy

⚠️ Estimated fetal weight alone is not a clinical decision tool. EFW is a is an estimateis not a measurement. In more than 30,000 singleton pregnancies born after the 22nd week of gestation, the ultrasound estimate performed no more than two days before birth fell within ±10% of the actual birth weight in only 67–76% of cases, depending on the formula used; The rate of staying in the ±20% band is 94-98% (UpToDate, Prenatal assessment of gestational age, date of delivery, and fetal weight). When making a decision, the percentile, the course of the growth curve, amniotic fluid, Doppler findings and the clinical picture are evaluated together.

📋 Method and Resources

What does this tool calculate: Calculates estimated fetal weight (EFW) in grams from ultrasound biometry (BPD, HC, AC, FL) with the Hadlock regression equation; If the gestational week is entered, it shows a rough percentile position according to the Hadlock 1991 in utero weight standard.
Formulas used: Four-parameter (BPD + HC + AC + FL): log 10 (EFW) = 1.3596 + 0.0064 × HC + 0.0424 × AC + 0.174 × FL + 0.00061 × BPD × AC − 0.00386 × AC × FL Three-parameter (HC + AC + FL): log 10 (EFW) = 1.326 + 0.0107 × HC + 0.0438 × AC + 0.158 × FL − 0.00326 × AC × FL In both equations, EFW is in grams, all measurements are in centimeters (the form on the page takes millimeters and converts). Source: Hadlock FP et al., Am J Obstet Gynecol 1985;151:333-337.
Which Hadlock variant: Multiple models have been published based on the combination of measurements in Hadlock's 1985 study. This page uses the four-parameter model if BPD is entered, otherwise it uses the head-abdominal-femur (three-parameter) model and writes which one was used in the result. UpToDate recommends using the modified Hadlock formula, which requires measurement of the head, abdomen, and femur; the same source states that the Hadlock method, which includes BPD, appears to be more effective than the INTERGROWTH-21st method in estimating birth weight and detecting extreme percentiles.
How is the percentile calculated?: Hadlock FP, Harrist RB, Martinez-Poyer J. Radiology 1991;181:129-133 in utero weight standard: mean weight = e ^{(0.578 + 0.332 × GH − 0.00354 × GH²)} (GH = weeks of gestation), standard deviation = 12.7% of the mean (constant throughout pregnancy in original publication ±12.7% variance reported). The percentile is calculated assuming a normal distribution and is a rough estimate.
In which weeks is it available?: Ultrasounds for EFW are generally not useful before approximately 24 weeks of gestation (UpToDate). On this page, percentile calculation is limited to 20–40 weeks, taking into account the range covered by the Hadlock 1991 standard.
Limitations: Formulas are most accurate near term and within the normal weight range. The weight is more often underestimated in larger babies and overestimated more often in smaller babies. In a systematic review comparing eleven formulas, no formula was found to be consistently superior, with random error exceeding 14% of birth weight in 5% of cases. Accuracy in twins is lower than in singleton pregnancies (±10% band: singleton 62%, twin 50%). These formulas are not used in anomalies that impair measurement, such as abdominal wall defects, severe hydrocephalus, microcephaly, and skeletal dysplasia.
Data privacy: The measurements you enter are processed only in your browser, they are not sent to the server and are not stored.

Last clinical update: 2026-09-11 Related page: Anomaly scan →

📐 How is EFW calculated?

The estimated fetal weight is not obtained by directly weighing the baby, but by combining several dimensions measured on ultrasound in a regression equation. At least 30 different formulas have been developed to date; The most commonly used ones are the Warsof formula with Shepard correction and the Hadlock formulas (UpToDate). Hadlock equations are included in the biometry package of almost all ultrasound devices.

The equations used in the calculation on this page are taken from the 1985 prospective study by Hadlock et al ( Am J Obstet Gynecol 1985;151:333-337 ). The equations are written clearly in the "Method and Resources" section above; Depending on what measurements you enter, a four-parameter or three-parameter model is applied, and the result indicates which one is used.

🎯 Accuracy — in numbers

The most misunderstood aspect of EFW is its accuracy. The following values are taken from UpToDate's Prenatal assessment of gestational age, date of delivery, and fetal weight topic:

Concordance of ultrasound EFW with actual birth weight
Criteria	Finding
EFW remains within ±10% of birth weight	67-76% (varies by formula)
EFW remains within ±20% of birth weight	94%-98
Random error exceeds 14% of birth weight	In 5% of cases
Macrosomia (birth weight >4000 g) prediction by Hadlock method	Positive likelihood ratio 5.7 (95% CI 4.3-7.6); negative likelihood ratio 0.48 (95% CI 0.38-0.60)
Staying within ±10% band — singleton and twin comparison	Singleton 62%, twin 50%

In short: an EFW result indicates a band of several hundred grams around the baby's actual weight, not a single exact number. If the same baby is measured by two different people on the same day, two different EFWs may occur; Intraobserver and interobserver variability were evident in these publications.

📈 Why is percentile alone not enough?

The meaning of an EFW value is where it stands relative to the expected weight at that gestational age. However, UpToDate emphasizes that when diagnosing growth restriction, the EFW or AC percentile in a single examination can be misleading, and the course of the growth curve and the change between the two measurements should also be evaluated. For example, steady growth along the 10th percentile line is reassuring, while a decline from the 60th percentile to the 20th percentile is alarming.

Also, which growth standard is used changes the result: Hadlock reference and INTERGROWTH-21st standard may give different percentiles for the same baby; The success of the standards in detecting small and large babies is also different. Minor discrepancies have also been reported between reproduced versions of the Hadlock 1991 chart (Gleason JL, A comment on revisiting Hadlock, Am J Obstet Gynecol 2026). Therefore, the percentile on this page is a rough indicator of position; It does not replace the growth curve used in your clinic.

🧾 How to read the result?

Consider a range, not a single number. The ±10% band shown in the result is a reminder of where the measurement typically falls relative to actual weight.
The quality of the measurements is the quality of the result. An AC measurement taken in the wrong plane shifts the entire calculation; AC is the most weighted parameter in the equations.
If there is an anomaly, the formula changes. In cases of abdominal wall defects, severe ventriculomegaly or skeletal dysplasias, a formula that does not include the relevant measurement is preferred. Large AC due to an intra-abdominal mass or ascites is an exception to this rule: the weight gain here is real and the AC is included in the calculation.
Accuracy increases as the time of birth approaches, but at extreme weights (very small or very large babies) the margin of error still increases.

📚 Sources of Information on This Page

Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK — Estimation of fetal weight with the use of head, body, and femur measurements: a prospective study. Am J Obstet Gynecol 1985;151(3):333-337. Original source of EFW equations on this page. PMID 3881966: doi.org
Hadlock FP, Harrist RB, Martinez-Poyer J — In utero analysis of fetal growth: a sonographic weight standard. Radiology 1991;181(1):129-133. In utero weight standard used in percentile calculation; A constant ±12.7% (1 SD) variance was reported throughout pregnancy. PMID 1887021: doi.org
UpToDate — Prenatal assessment of gestational age, date of delivery, and fetal weight (topic version 74.0, last updated 21 November 2025; literature review August 2026): formula selection, accuracy rates, twin and anomaly situations, limits of percentile interpretation: uptodate.com
Gleason JL — A comment on revisiting Hadlock. Am J Obstet Gynecol 2026. Methodological commentary noting inconsistencies between reproduced versions of the Hadlock 1991 table. PMID 40602587: doi.org

Last updated: September 11, 2026. This tool is aimed at healthcare professionals and pregnant women who want to be informed; Estimated fetal weight alone is not the basis for clinical decision making.