Calculate estimated fetal weight from ultrasound biometry (BPD, HC, AC, FL) with the Hadlock formula, and check the growth 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:
log10(EFW) = 1.3596 + 0.0064*HC + 0.0424*AC + 0.174*FL + 0.00061*BPD*AC - 0.00386*AC*FL
The estimated weight in grams is then found by raising 10 to that power:
EFW = 10^(log10(EFW))
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
If you do not have all four measurements, the calculator switches to the matching Hadlock equation. Each one drops the parameters you are missing:
Hadlock 1: log10(EFW) = 1.304 + 0.05281*AC + 0.1938*FL - 0.004*AC*FL Hadlock 2: log10(EFW) = 1.335 + 0.0316*BPD + 0.0457*AC + 0.1623*FL - 0.0034*AC*FL Hadlock 3: log10(EFW) = 1.326 + 0.0107*HC + 0.0438*AC + 0.158*FL - 0.00326*AC*FL
The “Measurements you have” menu picks which equation runs, so you only see the input fields that equation needs. You can type values in centimeters or millimeters; millimeters are converted to centimeters before the formula is applied. The result is shown in grams, kilograms, and pounds and ounces.
The second mode, “Weight percentile for gestational age,” does not use the weight formula. It compares an estimated weight you already have against a Hadlock growth reference for that week. The calculator reads the 10th, 50th, and 90th percentile weights for the gestational age, estimates the spread from that band, and reports where the weight falls:
z = (EFW - median) / SD SD = (90th percentile - 10th percentile) / 2.563
- 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 lists Hadlock reference weights in grams for the 10th, 50th, and 90th percentiles at selected weeks, with the median also shown in pounds and ounces. The optional gestational age field and the percentile mode both read from this same reference, so you can check whether an estimated weight sits inside the typical range for the week.
| Gestational age | 10th percentile | 50th percentile (median) | 90th percentile | Median (lb, oz) |
|---|---|---|---|---|
| 16 weeks | 121 g | 146 g | 171 g | 0 lb 5 oz |
| 20 weeks | 275 g | 331 g | 387 g | 0 lb 12 oz |
| 24 weeks | 556 g | 670 g | 784 g | 1 lb 8 oz |
| 28 weeks | 1004 g | 1210 g | 1416 g | 2 lb 11 oz |
| 32 weeks | 1621 g | 1953 g | 2285 g | 4 lb 5 oz |
| 36 weeks | 2335 g | 2813 g | 3291 g | 6 lb 3 oz |
| 40 weeks | 3004 g | 3619 g | 4234 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) = 3.220
EFW = 10^3.220 = 1658 g, which is about 1.66 kg or 3 lb 10 oz. Add a gestational age of 31 weeks 0 days and the tool places this near the 34th percentile, which is AGA.
Example 2: Check a percentile from a weight you already have.
Your report lists an EFW of 950 g at 28 weeks 0 days. The median for that week is 1210 g and the typical 10th to 90th range is 1004 to 1416 g. Because 950 g sits below the 10th percentile, the tool returns about the 5th percentile and flags it SGA.
Frequently Asked Questions
Which Hadlock formula does this calculator use?
By default it uses Hadlock 4, the four parameter equation that combines biparietal diameter, head circumference, abdominal circumference, and femur length. It is the most widely used version and lands within about 15 percent of actual birth weight roughly 85 percent of the time. If you are missing a measurement, pick the matching option and the calculator uses Hadlock 3 (HC, AC, FL), Hadlock 2 (BPD, AC, FL), or Hadlock 1 (AC, FL) instead.
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.