Estimated fetal weight below the 10th percentile defines a small-for-gestational-age (SGA) baby and may signal fetal growth restriction; above the 90th percentile is large-for-gestational-age. Ultrasound estimates can be off by 10–15%.
How to use the Fetal Weight Percentile Calculator
- Gestational age
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length (FL)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the gestational age and the three ultrasound measurements.
Tips
- Measure lying length for babies under 2 and standing height from age 2.
- Use the exact age in months for babies – a few weeks matters.
- Follow the trend across several visits; crossing two major percentile lines is more important than one value.
- Discuss concerns with your paediatrician.
Common mistakes to avoid
- Comparing a baby with the wrong sex chart.
- Reading the 50th percentile as “normal” and everything else as abnormal.
- Using corrected age incorrectly for premature babies.
Inputs explained
| Field | What to enter |
|---|---|
| Gestational age | In weeks |
| Head circumference (HC) | Units: cm, in, m, ft, mm |
| Abdominal circumference (AC) | Units: cm, in, m, ft, mm |
| Femur length (FL) | Units: cm, in, m, ft, mm |
What each result means
- Estimated fetal weight = 10^(1.326 – 0.00326×[Abdominal circumference (AC)]×[Femur length (FL)] + 0.0107×[Head circumference (HC)] + 0.0438×[Abdominal circumference (AC)] + 0.158×[Femur length (FL)])
- Average weight at this gestation = exp(0.578 + 0.332×[Gestational age] – 0.00354×[Gestational age]^2)
- Fetal weight percentile = ncdf(([Estimated fetal weight] – [Average weight at this gestation])÷(0.127×[Average weight at this gestation]))×100
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
log₁₀ EFW (g) = 1.326 − 0.00326 × AC × FL + 0.0107 × HC + 0.0438 × AC + 0.158 × FL (cm)
Mean weight for GA: ln W = 0.578 + 0.332 × GA − 0.00354 × GA²; SD ≈ 12.7% of the mean
Percentile = Φ((EFW − mean) ÷ SD)
Worked example
Inputs — Gestational age: 30 weeks; Head circumference (HC): 27.5 cm; Abdominal circumference (AC): 25.5 cm; Femur length (FL): 5.7 cm.
Results — Estimated fetal weight = 1,459 g; Average weight at this gestation = 1,559 g; Fetal weight percentile = 31 %; Category = Appropriate for gestational age.
Key terms
- EFW
- Estimated fetal weight.
- SGA/LGA
- Small/large for gestational age.
Frequently asked questions
What percentile is small for a baby?
Below the 10th percentile is small for gestational age.
How accurate is estimated fetal weight?
Usually within 10–15% of the actual birth weight.
Is a small baby always unhealthy?
No – many are constitutionally small; Doppler and growth over time help tell the difference.