Birthweight Percentile Calculator
Estimate newborn birthweight percentile from gestational age and weight at delivery.
What Is a Birthweight Percentile?
A birthweight percentile compares a newborn's weight to a reference population at the same gestational age. A 50th percentile means half of reference infants weigh less and half weigh more. Related tools include the Baby Percentile Calculator for postnatal growth tracking.
How Percentiles Are Calculated
This calculator uses Fetal Medicine Foundation reference medians by gestational week with linear interpolation between weeks. The z-score is $(weight - median) / SD$, and the percentile comes from the normal cumulative distribution.
Birthweight Classifications
Extremely low birth weight (ELBW) is below 1,000 g. Very low birth weight (VLBW) is below 1,500 g. Low birth weight (LBW) is below 2,500 g. Normal birth weight ranges from 2,500 to 4,000 g. High birth weight (HBW) is above 4,000 g.
Frequently Asked Questions
Which reference chart does this calculator use?
It uses approximate FMF fetal and neonatal population weight charts with median and spread values by gestational week from 22 to 41 weeks.
Can I enter decimal gestational weeks?
Yes. Enter gestational age with decimals such as 35.7 weeks. The calculator interpolates reference values between whole weeks.
What is a normal birthweight percentile?
Percentiles between about the 10th and 90th can be normal depending on clinical context. The 50th percentile is the population median, not a target weight.
Does preterm birth affect interpretation?
Yes. Preterm infants are compared to references at their gestational age at birth, not their due date. Very preterm births need specialist interpretation.
Should I use grams or pounds?
Enter birth weight in grams for this calculator. One pound equals approximately 454 grams.
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