Birth Weight Percentile Calculator
Assess newborn size for gestational age. Based on sex-specific population data from 24 to 42 weeks.
Use completed weeks (e.g., 37.5 for 37 weeks + 3 days).
We convert to grams to compare with the reference chart.
How to Use This Calculator
Confirm gestational age
Use the best obstetric estimate (first trimester ultrasound or IVF dating). Accurate dating is essential for meaningful results.
Measure birth weight
Prefer the delivery room scale prior to fluid loss. Enter the weight and choose grams, kilograms, or pounds.
Interpret percentile
Use percentiles to screen for SGA/LGA but correlate with maternal health, placenta findings, and newborn physical exam.
Formula
1. Interpolate 10th, 50th, and 90th percentile weights for the chosen gestational age.
The reference values come from sex-specific population curves widely used in neonatology. They approximate the INTERGROWTH-21st and CDC datasets.
About Birth Weight Percentiles
Birth weight percentiles are a quick way to categorize newborn size. Small-for-gestational-age (SGA) infants are at higher risk for hypoglycemia, hypothermia, and feeding difficulties. Large-for-gestational-age (LGA) infants may experience shoulder dystocia or neonatal hypoglycemia, especially in diabetic pregnancies.
Modern growth standards adjust for sex because boys tend to weigh slightly more than girls at any gestational age. Further adjustments (e.g., ethnicity, parity, maternal BMI) exist but are beyond the scope of this calculator.
Clinical pearls
- Use corrected gestational age for preterm or post-term infants when tracking postnatal growth.
- Evaluate placenta and umbilical cord findings when SGA is suspected.
- Consider maternal diabetes, genetics, and ultrasound estimates when interpreting LGA.
Frequently Asked Questions
What if gestational age is uncertain?
Use the best clinical estimate, often based on early ultrasound. If dating is uncertain, interpret percentiles cautiously and rely on the clinical exam.
Do twins use the same chart?
Twins typically follow different growth trajectories. Specialized twin growth charts may provide better context for multiples.
How soon should I recheck weight?
Newborns normally lose up to 7–10% of birth weight in the first week and regain it by day 10–14. Follow your pediatrician's schedule for monitoring.
Can SGA babies catch up?
Many SGA infants achieve catch-up growth within the first 2 years, especially with adequate nutrition. Persistent failure to thrive warrants investigation.
Why choose sex-specific curves?
Boys weigh slightly more than girls at every gestational age. Using sex-specific charts avoids misclassifying healthy babies as SGA or LGA.