Birth Weight Percentile Calculator
Calculate birth weight percentile by gestational age and sex, classifying babies as SGA, AGA or LGA.
What is Birth Weight Percentile?
Birth weight is one of the strongest single predictors of newborn health. Babies born below the 10th percentile are classified as Small for Gestational Age (SGA) and may have higher risks of hypoglycaemia, temperature instability and longer feeding establishment. Above the 90th percentile, babies are Large for Gestational Age (LGA) — increasing the risk of birth trauma, shoulder dystocia and neonatal hypoglycaemia. Most babies (80%) fall between, classified as Appropriate for Gestational Age (AGA).
How is it calculated?
The calculator uses gestational-age and sex-specific birth-weight means and standard deviations approximated from the Fenton 2013 international growth chart. A z-score is computed and converted to a percentile via the normal CDF. Use clinical charts for definitive classification.
Frequently Asked Questions
- What is SGA?
- Birth weight below the 10th percentile for gestational age.
- Is LGA dangerous?
- Risks rise modestly above the 90th percentile, especially during delivery.
- Does this work for twins?
- No — multifetal pregnancies have different growth curves.
- Can preterm babies be SGA?
- Yes — SGA reflects size for age, regardless of when delivery occurred.