What does birth weight percentile mean?

A birth weight percentile tells you how your baby's weight compares to other babies born at the same gestational age and of the same sex. A baby at the 50th percentile weighs exactly the median for their gestational age. A baby at the 10th percentile weighs more than 10% of babies at the same gestational age. Percentile bands below 10 (small for gestational age, SGA) or above 90 (large for gestational age, LGA) may be reviewed by your midwife or doctor, though many babies in these ranges are perfectly healthy.

Which reference chart does this calculator use?

This calculator uses the INTERGROWTH-21st international newborn size standards published by researchers at the University of Oxford (Villar et al., The Lancet, 2014). These are the most widely used international standards for birth weight by gestational age and sex, covering 24 to 42 completed weeks. The method uses a Box-Cox z-score transformation with published median (mu), coefficient of variation (sigma), and skewness (lambda) parameters, converted to a percentile via the standard normal distribution.

What is considered a normal birth weight in New Zealand?

For full-term babies (37 to 42 weeks gestation), a normal birth weight in New Zealand is generally between 2,500 g and 4,500 g, with the average around 3,300 to 3,450 g for males and slightly less for females. The percentile system accounts for gestational age, making it a more accurate measure than absolute weight alone. A premature baby at 28 weeks would be expected to weigh around 1,100 to 1,300 g at the median, while a term baby at 40 weeks would be expected to weigh around 3,300 to 3,450 g.

Birth Weight Percentile Calculator

Find your newborn's birth weight percentile for their gestational age and sex. Enter the birth weight in grams and gestational age in completed weeks. Results use the INTERGROWTH-21st international newborn size standards (Villar et al. 2014).

A percentile below 10 is classified as small for gestational age (SGA). Above 90 is large for gestational age (LGA). Most babies between the 10th and 90th percentile are appropriate for gestational age (AGA).

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Updated June 2026 INTERGROWTH-21st newborn size standards (Villar et al., The Lancet, 2014).

1. Baby Details

Sex

Gestational age at birth (completed weeks)

Birth weight

grams

2. Reference Percentiles (your gestational age)

3rd percentile-

10th percentile (SGA threshold)-

25th percentile-

50th percentile (median)-

75th percentile-

90th percentile (LGA threshold)-

97th percentile-

Birth Weight Percentile Result

Birth Weight

Grams entered

Percentile
-
For gestational age and sex

Z-Score

Standard deviations from median

Classification

SGA / AGA / LGA

Percentile Position

Below 10th 10th-25th 25th-75th 75th-90th Above 90th

SGA

Low-Normal

Normal (AGA)

High-Normal

LGA

Measurement Details

Sex-

Gestational age-

Birth weight-

Reference median-

Z-score-

Percentile-

Weight in Kilograms

Birth weight (kg)-

Median for gestational age (kg)-

Difference from median-

3rd percentile weight-

10th percentile weight-

90th percentile weight-

Assessment: Enter weight and gestational age above.

INTERGROWTH-21st Reference Table (selected gestational ages)

Gestational Age	3rd %ile	10th %ile	50th %ile (median)	90th %ile	97th %ile

* Selected row matches your current gestational age and sex inputs. Females weigh slightly less at the median.

How Birth Weight Percentile Is Calculated

A birth weight percentile places a newborn's weight in context relative to all other babies born at the same gestational age and of the same sex. The calculation uses the INTERGROWTH-21st newborn size standards (Villar et al. 2014), the most widely accepted international reference for this purpose.

The method uses a Box-Cox power transformation (LMS method). For each sex and gestational age in completed weeks from 24 to 42, the reference provides three parameters:

Mu (M): the median birth weight in grams
Sigma (S): the coefficient of variation (standard deviation divided by mean)
Lambda (L): the Box-Cox power that normalises the skewed distribution

The z-score is computed as: Z = [(weight / M)^L - 1] / (L x S)

This z-score is then converted to a percentile using the cumulative distribution function of the standard normal distribution.

Worked Example

A male baby born at 40 weeks weighing 3,435 g (the default):

INTERGROWTH-21st male 40-week median (M): 3,435 g
Coefficient of variation (S): 0.1230
Box-Cox power (L): 0.35
Z = [(3435 / 3435)^0.35 - 1] / (0.35 x 0.1230) = [1 - 1] / 0.04305 = 0.00
Percentile = phi(0.00) = 50.0th percentile
Classification: AGA (appropriate for gestational age)

This matches the calculator's default output of the 50th percentile for a 3,435 g male at 40 weeks.

Classification Thresholds

Classification	Percentile Range	Meaning
Very small for gestational age (VSGA)	Below 3rd	Weight below the 3rd centile; clinical review recommended
Small for gestational age (SGA)	3rd to below 10th	Weight below the 10th centile; may warrant monitoring
Appropriate for gestational age (AGA)	10th to 90th	Normal range for gestational age
Large for gestational age (LGA)	Above 90th to 97th	Weight above the 90th centile; may warrant monitoring
Very large for gestational age (VLGA)	Above 97th	Weight above the 97th centile; clinical review recommended

These thresholds are used by clinicians and midwives to identify babies who may benefit from additional monitoring. SGA does not always indicate a problem; some babies are small because their parents are small (constitutionally small). Clinical assessment always considers growth trends, Doppler findings, and family history.

Limitations

This calculator provides a reference centile based on international population data. It is not a diagnostic tool and does not replace clinical assessment by a midwife, obstetrician, or neonatologist. Actual clinical classifications in New Zealand may use locally customised growth charts (such as the GROW customised centile system), which adjust for maternal height, weight, parity, and ethnicity. If you have concerns about your baby's weight or growth, speak with your LMC, midwife, or GP.

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Sources and method: Villar J, Cheikh Ismail L, Victora CG, et al. (2014). International standards for newborn weight, length, and head circumference by gestational age and sex: the INTERGROWTH-21st Newborn Cross-Sectional Study. The Lancet 384(9946):857-868. INTERGROWTH-21st project (intergrowth21.tghn.org). Ministry of Health New Zealand, Well Child Tamariki Ora programme.

This calculator provides reference centiles for informational purposes only. It is not a substitute for professional clinical assessment. Centiles use INTERGROWTH-21st international standards, which may differ from locally customised growth charts used in clinical practice in New Zealand. Always discuss your baby's growth with your midwife, LMC, or doctor.