Calculate newborn weight percentile by gestational age and sex. Classifies as SGA, AGA, or LGA with z-scores and growth reference data.
The Birthweight Percentile Calculator determines where a newborn's weight falls relative to population norms for gestational age and sex. Using established growth reference data, it classifies infants as Small for Gestational Age (SGA, <10th percentile), Appropriate for Gestational Age (AGA, 10th-90th percentile), or Large for Gestational Age (LGA, >90th percentile).
Birthweight percentile is one of the most important neonatal assessments because it helps identify infants at risk for hypoglycemia, hypothermia, respiratory distress, and longer-term metabolic consequences. SGA infants may have experienced intrauterine growth restriction, while LGA infants are associated with maternal diabetes and increased birth injury risk.
This calculator provides z-scores, percentiles, comparison to median expected weight, and the Ponderal Index for body proportionality assessment to support neonatal evaluation at birth. Interpretation still depends on correct gestational dating, scale measurement, and the nursery reference standard your team uses.
Birthweight percentile is useful because it places a newborn's size into gestational-age context instead of relying on raw grams alone. That helps frame nursery review, documentation, and follow-up discussions in a more standardized way.
Using percentile-based classification also makes it easier to compare the newborn with the growth reference your team uses without pretending the percentile alone decides every monitoring or feeding pathway.
Z-Score = ((Weight/Median)^L - 1) / (L × S) [LMS method] Where L, M, S are age- and sex-specific parameters from growth reference data. Percentile derived from z-score using standard normal distribution. Ponderal Index = (Weight in grams / Length in cm³) × 100
Result: 48th percentile — Appropriate for GA
A male infant born at 40 weeks weighing 3400 g is very close to the median (3470 g) for his gestational age, placing him at approximately the 48th percentile — solidly in the AGA range.
SGA infants have higher rates of hypoglycemia, hypothermia, feeding difficulty, and other early transitional issues, but percentile alone does not prove pathologic growth restriction. It is one part of a broader newborn assessment.
LGA classification signals that the infant is larger than expected for gestational age. It helps frame review of maternal diabetes history, birth course, and newborn adaptation, but the percentile itself does not replace the rest of the nursery evaluation.
Multiple growth references exist. The Fenton charts are commonly used in preterm infants, while WHO and Intergrowth-21st standards are more common around term. Because classification can shift with the reference used, this page works best as a structured comparison aid rather than as a stand-alone nursery protocol.
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This calculator estimates birthweight percentile from sex, gestational age, and birth weight using an interpolated LMS-style reference table, then reports the corresponding percentile, z-score, and SGA/AGA/LGA band. It is intended as a quick nursery-reference estimate rather than a claim to reproduce every hospital chart exactly.
The output depends heavily on accurate gestational dating and on which newborn-size standard a nursery actually uses. Because different services may follow Fenton, INTERGROWTH-21st, or other local references, the result should be treated as a screening summary and reconciled with the unit's official chart when classification matters clinically.
SGA is defined as birthweight below the 10th percentile for gestational age and sex. Some clinicians use a stricter cutoff of <3rd percentile for severe SGA or <2 standard deviations below the mean.
Not exactly. SGA is a statistical classification based on birth weight. IUGR implies a pathological process that prevented the fetus from reaching its growth potential. An SGA infant may be constitutionally small, while an AGA infant may have experienced IUGR but still be above the 10th percentile.
The most common cause is maternal diabetes (gestational or pre-existing). Other factors include maternal obesity, excessive gestational weight gain, multiparity, and genetic predisposition. LGA infants are at increased risk for shoulder dystocia, birth injuries, and neonatal hypoglycemia.
This calculator uses approximated LMS parameters based on population-level data. For clinical purposes, Fenton (preterm) and WHO or Intergrowth-21st (term) standards are commonly used. Always verify with your institution preferred reference.
Accuracy depends on correct gestational age dating. Ultrasound dating in the first trimester is most accurate (±5 days). Incorrect dating can significantly affect percentile classification, especially near the SGA/AGA and AGA/LGA boundaries.
The Ponderal Index (PI = weight/length³ × 100) measures body proportionality. A low PI indicates a thin, wasted infant (asymmetric growth restriction), while a high PI suggests relative overweight. Normal range is approximately 2.0-2.85.