Calculate estimated fetal weight from ultrasound BPD, HC, AC and FL with the Hadlock formula, and see the fetal weight percentile for gestational age.
Estimated Fetal Weight (Hadlock) Formula
The default mode uses the four parameter Hadlock 4 equation. It takes the biparietal diameter, head circumference, abdominal circumference, and femur length, all in centimeters, and returns the base 10 logarithm of the weight in grams:
The estimated weight in grams is then found by raising 10 to that power:
Variables:
- EFW is the estimated fetal weight in grams
- BPD is the biparietal diameter in centimeters
- HC is the head circumference in centimeters
- AC is the abdominal circumference in centimeters
- FL is the femur length in centimeters
Choose the measurement set in the “Find using” menu. Each equation uses only the listed measurements:
The “Find using” menu picks the equation and shows its required inputs. Each measurement accepts centimeters or millimeters, with values converted when you change units. The result is shown in grams, kilograms, and pounds and ounces. In EFW modes, gestational age is optional: blank weeks omit percentile; blank additional days mean zero.
The “Percentile from reported EFW” mode compares a reported estimated weight with the equation-based Hadlock 1991 reference. It uses a normal distribution of log weight, rather than interpolating the original table. This equation-based reference differs from the original tabulated chart, which has documented inconsistencies. Results may differ from a clinic’s chosen chart. The supported reference range is 14 weeks 0 days through 40 weeks 0 days; ages outside it are rejected rather than clamped.
- Below the 10th percentile is flagged SGA, small for gestational age
- Between the 10th and 90th percentile is AGA, appropriate for gestational age
- Above the 90th percentile is flagged LGA, large for gestational age
Fetal Weight Percentiles by Week
This table is calculated from the same equation-based Hadlock reference as the tool, with weights rounded to grams. The 10th–90th interval is a population reference band, not an error range for an individual ultrasound estimate. It is not the original Hadlock tabulated chart.
| Gestational age | 10th percentile | 50th percentile (median) | 90th percentile | Median (lb, oz) |
|---|---|---|---|---|
| 16 weeks | 125 g | 146 g | 170 g | 0 lb 5 oz |
| 20 weeks | 284 g | 331 g | 386 g | 0 lb 12 oz |
| 24 weeks | 574 g | 670 g | 781 g | 1 lb 8 oz |
| 28 weeks | 1038 g | 1210 g | 1412 g | 2 lb 11 oz |
| 32 weeks | 1674 g | 1953 g | 2277 g | 4 lb 5 oz |
| 36 weeks | 2412 g | 2813 g | 3281 g | 6 lb 3 oz |
| 40 weeks | 3103 g | 3619 g | 4221 g | 8 lb 0 oz |
Percentile alone does not tell the whole story. The category below is how that percentile is usually described on a growth chart.
| Category | Percentile range | What it suggests |
|---|---|---|
| SGA (small for gestational age) | Below the 10th | Smaller than about 90 percent of fetuses at this age; often prompts closer growth monitoring |
| AGA (appropriate for gestational age) | 10th to 90th | Within the typical range for this age |
| LGA (large for gestational age) | Above the 90th | Larger than about 90 percent of fetuses at this age; may prompt a check for causes such as gestational diabetes |
Example Problems
Example 1: Estimate fetal weight from all four measurements.
A scan reports BPD 8.0 cm, HC 29.0 cm, AC 27.0 cm, and FL 5.7 cm. Using Hadlock 4:
log10(EFW) = 1.3596 + 0.0064(29.0) + 0.0424(27.0) + 0.174(5.7) + 0.00061(8.0)(27.0) – 0.00386(27.0)(5.7) ≈ 3.219506
EFW = 103.219506 ≈ 1658 g, about 1.658 kg or 3 lb 10 oz. At 31 weeks 0 days, the equation-based reference places the unrounded estimate near the 32.4th percentile, within the AGA reference band.
Example 2: Check a percentile from a weight you already have.
An EFW of 950 g at 28 weeks 0 days is approximately the 2.2th percentile on this equation-based reference. Its median is about 1210 g and its 10th–90th interval about 1038–1412 g. This lies in the SGA reference band; it does not by itself diagnose growth restriction.
Frequently Asked Questions
Which Hadlock formula does this calculator use?
By default it uses the four-parameter equation with BPD, HC, AC and FL. Select another measurement set to use the matching three- or two-parameter equation. Numbered Hadlock conventions vary, so identify the equation by its measurements. Errors of 10–15 percent are common and may be larger; the estimate is not an exact birth weight.
What counts as a normal fetal weight percentile?
A weight between the 10th and 90th percentile for the gestational age is appropriate for gestational age (AGA). Below the 10th is small for gestational age (SGA) and above the 90th is large for gestational age (LGA). A single percentile is not a diagnosis. Clinicians look at the trend across several scans, amniotic fluid, Doppler studies, and your history before drawing conclusions.
Is estimated fetal weight the same as birth weight?
No. Estimated fetal weight is a prediction from ultrasound measurements, not a direct reading. Even with good technique it carries an error of roughly 10 to 15 percent, and the gap can be wider for very small or very large babies. Use it to follow growth over time rather than as an exact birth weight.
