Birth Weight Percentile Calculator
Enter your baby's birth weight, gestational age in weeks, and sex to get an instant birth weight percentile based on the Olsen 2010 US intrauterine growth reference. The result tells you where your baby's weight falls among other infants born at the same gestational age, and whether the weight is classified as Small for Gestational Age (SGA), Appropriate for Gestational Age (AGA), or Large for Gestational Age (LGA). Switch between metric and imperial units anytime.
What is a birth weight percentile?
A birth weight percentile tells you how a newborn's weight compares to other infants born at exactly the same gestational age and of the same sex. A percentile of 50 means the baby weighs more than half of comparable infants and less than the other half. A percentile of 10 means the baby weighs more than only 10% of peers, placing them in the Small for Gestational Age (SGA) category. A percentile of 90 means the baby outweighs 90% of comparable infants, placing them in the Large for Gestational Age (LGA) category. The range from the 10th to the 90th percentile is considered Appropriate for Gestational Age (AGA) and is the expected range for most healthy infants.
SGA, AGA, and LGA classifications
Small for Gestational Age (SGA) is defined as a birth weight below the 10th percentile for gestational age and sex. SGA can result from intrauterine growth restriction, maternal undernutrition, placental insufficiency, infection, or chromosomal factors. These babies require closer monitoring for hypoglycemia, hypothermia, and feeding difficulties. Large for Gestational Age (LGA) is defined as a birth weight above the 90th percentile and is associated with maternal diabetes, excessive gestational weight gain, and post-term delivery. LGA can increase the risk of shoulder dystocia, birth injury, and neonatal hypoglycemia. Appropriate for Gestational Age (AGA) infants fall between the 10th and 90th percentiles and are generally at lower risk from size-related complications, though prematurity itself carries independent risks regardless of percentile.
How gestational age affects birth weight percentiles
A 2,000 g baby born at 28 weeks of gestation (well above the median for 28 weeks) has a very different clinical picture than a 2,000 g baby born at 36 weeks (below the 10th percentile for 36 weeks). This is why percentile-based assessment is always gestational-age-specific: the same absolute weight can indicate growth restriction or normal growth depending on how many weeks gestation were completed. Full-term deliveries (37-41 weeks) use the same reference but place a higher absolute weight floor for normal range because babies grow rapidly in the final weeks. Preterm infants (under 37 weeks) are often assessed separately because their extrauterine environment differs from in-utero growth.
About the Olsen 2010 reference standard
This calculator uses the Olsen 2010 US intrauterine growth reference (Olsen IE et al., Pediatrics 2010;125:e214-e224), drawn from 391,681 infants born at 248 US hospitals in 33 states between 1998 and 2006; the final analytic sample was 257,855 singleton infants who survived to discharge. Separate LMS (Lambda-Mu-Sigma) parameter sets are provided for males and females at each completed gestational week from 23 to 41 weeks. The LMS method models the skewed distribution of birth weights via a Box-Cox power transformation, producing accurate percentile estimates even in the tails of the distribution. Other widely used references include the Fenton 2013/2025 charts (international meta-analysis), INTERGROWTH-21st, and the WHO Multicentre Growth Reference. The Olsen reference is most commonly used in US neonatal clinical practice for birth weight classification.
WHO birth weight categories
| Category | Birth weight | Clinical significance |
|---|---|---|
| ELBW - Extremely Low Birth Weight | Under 1,000 g (2 lb 3 oz) | Requires NICU; very high risk |
| VLBW - Very Low Birth Weight | 1,000-1,499 g (2-3.3 lb) | Requires intensive care |
| LBW - Low Birth Weight | 1,500-2,499 g (3.3-5.5 lb) | Increased monitoring needed |
| NBW - Normal Birth Weight | 2,500-4,000 g (5.5-8.8 lb) | Expected range at term |
| HBW - High Birth Weight | Over 4,000 g (8.8 lb) | Increased delivery risk |
World Health Organization definitions used worldwide for clinical triage of newborns.
Frequently asked questions
What is a normal birth weight percentile?
A birth weight between the 10th and 90th percentile for the baby's gestational age and sex is considered Appropriate for Gestational Age (AGA) and is the expected range for most healthy newborns. The 50th percentile (median) simply means average weight for that gestational age. There is no single "ideal" percentile; what matters most is whether the weight is appropriate for how many weeks the pregnancy lasted.
What does SGA mean for my baby?
SGA (Small for Gestational Age) means the baby's birth weight is below the 10th percentile compared to other babies born at the same gestational age and sex. SGA itself is not a diagnosis; it is a statistical classification that prompts closer monitoring for low blood sugar, difficulty maintaining body temperature, feeding challenges, and occasionally underlying causes like placental insufficiency. Most SGA babies do well with appropriate care.
Is birth weight percentile the same as baby growth chart percentile?
No. Birth weight percentile compares weight to babies born at the same gestational age (from 23 to 41 weeks). Postnatal growth charts (used at well-child visits) compare the baby's weight, length, and head circumference to other babies of the same age after birth, with age often corrected for prematurity. This calculator applies only at the moment of birth.
Why does my baby's sex matter for the percentile?
Male and female newborns grow at slightly different rates in utero, with male infants averaging somewhat higher birth weights than female infants at the same gestational age. Using sex-specific reference curves ensures the percentile comparison is fair: a 3,300 g boy at 38 weeks may be at a different percentile than a 3,300 g girl at 38 weeks. This calculator uses separate LMS parameter tables for each sex.
What is the average birth weight at full term?
In the Olsen 2010 US reference the median (50th percentile) birth weight at 40 weeks is 3,579 g (about 7 lb 14 oz) for boys and 3,454 g (about 7 lb 10 oz) for girls. The 10th percentile at 40 weeks is 2,950 g for boys and 2,856 g for girls, and the 90th percentile is 4,233 g and 4,069 g respectively. Weights between about 2,500 g (5 lb 8 oz) and 4,000 g (8 lb 13 oz) are considered normal by WHO definitions regardless of sex, though the percentile-based AGA range gives a more gestational-age-specific picture.
Can I use this calculator for twins or multiples?
This calculator uses singleton birth weight reference data (Olsen 2010). Twins and higher-order multiples have systematically lower birth weights than singletons at the same gestational age, so applying singleton references to multiples will overestimate their percentile. Dedicated twin growth references exist and should be used for multiples; your neonatologist or obstetrician can advise on the appropriate standard.
Sources
- Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New intrauterine growth curves based on United States data. Pediatrics. 2010;125(2):e214-e224. doi:10.1542/peds.2009-0913 (source of the LMS parameters used here).
- American Academy of Pediatrics. Intrauterine Growth Curves (Olsen 2010) - printable charts, including the SGA <10th / AGA 10th-90th / LGA >90th birth size definitions.
- World Health Organization. International statistical classification of diseases and related health problems: low birth weight definitions. WHO, Geneva.
- Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatrics. 2013;13:59.