Fetal weight chart by week and the Hadlock formula: values, calculation and use in the report
The Hadlock 1991 fetal weight chart week by week, the Hadlock formulas for calculating weight, a worked example and the differences from the WHO and INTERGROWTH-21st charts.
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This article is part of the guide Obstetric ultrasound: the three-trimester guide, from dating to Doppler.
The classic fetal weight chart is Hadlock 1991, in which the 50th percentile runs from 331 g at 20 weeks to 3,619 g at 40 weeks; each fetus's weight comes from a separate Hadlock formula (1985), calculated from the biometric measurements.
Fetal weight chart by week (Hadlock 1991)
| Weeks | P10 (g) | P50 (g) | P90 (g) |
|---|---|---|---|
| 20 | 275 | 331 | 387 |
| 21 | 331 | 399 | 467 |
| 22 | 398 | 478 | 559 |
| 23 | 471 | 568 | 665 |
| 24 | 556 | 670 | 784 |
| 25 | 652 | 785 | 918 |
| 26 | 758 | 913 | 1,068 |
| 27 | 876 | 1,055 | 1,234 |
| 28 | 1,004 | 1,210 | 1,416 |
| 29 | 1,145 | 1,379 | 1,613 |
| 30 | 1,294 | 1,559 | 1,824 |
| 31 | 1,453 | 1,751 | 2,049 |
| 32 | 1,621 | 1,953 | 2,285 |
| 33 | 1,794 | 2,162 | 2,530 |
| 34 | 1,973 | 2,377 | 2,781 |
| 35 | 2,154 | 2,595 | 3,036 |
| 36 | 2,335 | 2,813 | 3,291 |
| 37 | 2,513 | 3,028 | 3,543 |
| 38 | 2,686 | 3,236 | 3,786 |
| 39 | 2,851 | 3,435 | 4,019 |
| 40 | 3,004 | 3,619 | 4,234 |
Values transcribed from Table 1 of Hadlock 1991, which also includes weeks 10 to 19 and the 3rd and 97th percentiles. The chart came from 392 predominantly white, middle-class women.
Hadlock formula
With measurements in centimeters (BPD, HC, AC and FL) and the result in grams:
- Four measurements: log10(weight) = 1.3596 + 0.0064 HC + 0.0424 AC + 0.174 FL + 0.00061 BPD x AC - 0.00386 AC x FL. This is the model used to generate the 1991 chart.
- Three measurements: log10(weight) = 1.326 + 0.0107 HC + 0.0438 AC + 0.158 FL - 0.00326 AC x FL.
- 1991 chart: ln(weight) = 0.578 + 0.332 GA - 0.00354 GA², with GA in menstrual weeks. This gives the expected weight for age, not the weight of the fetus.
Fetal weight calculator example with three measurements: HC 33 cm, AC 33 cm and FL 7 cm give about 3,002 g. At 38 weeks, that weight lies between the Hadlock 10th (2,686 g) and 50th (3,236 g) percentiles.
Hadlock, WHO or INTERGROWTH-21st
- Hadlock 1991: an older descriptive reference from a single population. The 50th percentile column matches the equation at every week, but Roberts 2025 reported that the critical points of the equation and the table differ and advised preferring the equation over interpolating the table.
- WHO (Kiserud 2017): a longitudinal study in ten countries, with a chart from 14 to 40 weeks and EFW by the three-measurement Hadlock formula.
- INTERGROWTH-21st (Stirnemann 2017): a prescriptive standard from 22 to 40 weeks, with its own formula based on AC and HC.
The choice changes the classification. At 36 weeks, the 90th percentile is 3,291 g for Hadlock and 3,153 g for WHO: a 3,200 g fetus is large for gestational age by WHO only.
What the report should include
- EFW in grams, with percentile or Z-score.
- Formula and chart used, as ISUOG requests.
- Reference gestational age (early dating, with no re-dating by biometry).
- Margin of error: ISUOG cites common errors of 10 to 15%.
- Comparison with the previous scan, on the same chart.
Common mistakes
- Mixing formula and chart without checking which formula the chart was built on.
- Plotting EFW on a birth-weight chart.
See also our articles on fetal biometry, planes and charts and fetal growth restriction.
How Laudário helps
In the obstetric templates of Laudário:
- Weight is calculated with the best Hadlock formula for the available measurements, and the report states which one was used.
- The percentile chart is chosen in Settings: Hadlock, FMF or INTERGROWTH-21st.
- Growth graphs follow the patient's scans.
Try it free for 15 days, no credit card required.
References
- Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology 1991;181:129-133.
- Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal weight with the use of head, body, and femur measurements: a prospective study. Am J Obstet Gynecol 1985;151:333-337.
- Roberts AW, et al. Revisiting the Hadlock 1991 population reference for estimated fetal weight. Am J Obstet Gynecol 2025.
- Kiserud T, et al. The World Health Organization fetal growth charts. PLoS Med 2017;14:e1002220.
- Stirnemann J, et al. International estimated fetal weight standards of the INTERGROWTH-21st Project. Ultrasound Obstet Gynecol 2017;49:478-486.
- Salomon LJ, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol 2019;53:715-723.
Physician with an RQE specialty registration in general ultrasound. Writes on the Laudário blog about reports, classifications and standardization. Who writes and how.
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