Estimated Fetal Weight (EFW): How It Is Calculated and Its Limits
Estimated fetal weight is not measured directly. It is calculated from biometric measurements using a regression formula. EFW is one of the most useful numbers in antenatal care, but it is an estimate with a real margin of error.
How EFW is calculated
Many formulas exist. The most widely used are the Hadlock formulas (1985), which combine different sets of BPD, HC, AC and FL. The WHO Fetal Growth Charts study calculated EFW with Hadlock's formula using HC, AC and FL:
(EFW in grams; HC, AC and FL in centimetres)
Most ultrasound machines calculate EFW automatically. The machine may be set to a different formula (for example a Hadlock version that also uses BPD). For consistency with the WHO EFW centiles, check which formula your machine uses.
This website does not calculate EFW. You enter the EFW reported by your ultrasound machine, and the tool plots it against the WHO EFW centiles.
How accurate is EFW?
Ultrasound EFW is commonly quoted to have an error of around 10–15% compared with the actual birth weight, and the error can be larger at the extremes of weight. For a fetus estimated at 3,000 g, the true weight could reasonably be anywhere from about 2,600 g to 3,400 g. Errors are larger when:
- the fetus is very small or very large (macrosomia),
- imaging is difficult (high maternal BMI, reduced liquor, fetal position),
- the head is engaged late in pregnancy, making HC hard to measure,
- AC is measured in an oblique plane, because AC carries the most weight in the formula.
Why AC matters most
AC is the component most affected by fetal nutrition and contributes most to the EFW calculation. A small error in AC changes EFW much more than a similar error in FL. When EFW looks unexpected, review the AC image first.
Using EFW on a growth chart
- Look at the trend, not one value. A single EFW below the 10th centile describes a small fetus. A series that falls across centiles describes slowing growth.
- Look at EFW together with AC. Both are used in the consensus definitions of FGR. If they disagree, look at the individual measurements.
- Allow enough time between scans. Because of the measurement error, EFW differences over less than about two weeks are hard to interpret.
- Twins: EFW is used to calculate inter-twin discordance. See Twin growth assessment.
WHO EFW reference values
Selected WHO EFW centiles (grams), from Table 11 of Kiserud et al. 2017:
| Weeks | 5th | 10th | 25th | 50th | 75th | 95th |
|---|---|---|---|---|---|---|
| 20 | 271 | 286 | 307 | 330 | 355 | 399 |
| 28 | 977 | 1,026 | 1,102 | 1,189 | 1,279 | 1,429 |
| 36 | 2,205 | 2,352 | 2,531 | 2,745 | 2,959 | 3,277 |
| 40 | 2,849 | 3,084 | 3,333 | 3,617 | 3,892 | 4,312 |
The full table and other parameters are in the original publication.
References
- 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.
- Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts: a multinational longitudinal study of ultrasound biometric measurements and estimated fetal weight. PLOS Med. 2017;14(1):e1002220. doi:10.1371/journal.pmed.1002220