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 their 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, as it helps identify infants at risk for hypoglycemia, hypothermia, respiratory distress, and long-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, supporting comprehensive neonatal evaluation at birth. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case. Use the example pattern when troubleshooting unexpected results.
Accurate birthweight classification guides immediate neonatal management decisions including blood glucose monitoring protocols, feeding strategies, temperature regulation, and observation intensity. It also triggers appropriate follow-up assessments and parental counseling.
Using standardized percentile calculations ensures consistent classification across providers and institutions, supporting evidence-based care pathways for newborns at the extremes of the growth spectrum.
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 face immediate risks including hypoglycemia, polycythemia, hypothermia, and feeding difficulties. Long-term, they have increased risk of neurodevelopmental delays, metabolic syndrome, cardiovascular disease, and shorter adult stature. The Barker hypothesis proposes that fetal undernutrition programs adult disease through epigenetic mechanisms.
LGA infants are at risk for birth trauma (shoulder dystocia, brachial plexus injury, clavicle fracture), hypoglycemia (especially with maternal diabetes), hyperbilirubinemia, and respiratory complications. Delivery planning for suspected macrosomia may include consideration of cesarean delivery when estimated fetal weight exceeds 4500g in diabetic mothers or 5000g in non-diabetic mothers.
Multiple growth references exist. The Fenton charts (2013) are standard for preterm infants, transitioning to WHO curves at term. The Intergrowth-21st project provides international standards. Choice of reference affects classification rates — clinicians should know which reference their institution uses.
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.