Birthweight Percentile Calculator - Newborn Growth Assessment
Calculate newborn birth weight percentile by gestational age using WHO and INTERGROWTH-21st growth standards. Check if your baby weight is appropriate for gestational age with SGA/AGA/LGA classification.
About This Calculator
The Birthweight Percentile Calculator – Newborn Growth Assessment helps parents and healthcare providers evaluate a newborn's weight relative to other babies born at the same gestational age. By comparing the baby's birth weight to population-based reference data, the calculator determines the weight percentile — a key metric for identifying infants who may need closer monitoring or follow-up care.
Our calculator computes a Z-score from the baby's weight and the average weight for that gestational week, using a coefficient of variation of 15% to estimate the standard deviation. The Z-score is then converted to a percentile using the standard logistic approximation. Results classify the baby as Small for Gestational Age (SGA, below 10th percentile), Appropriate for Gestational Age (AGA, 10th–90th percentile), or Large for Gestational Age (LGA, above 90th percentile). Reference data is based on INTERGROWTH-21st and Fenton growth standards for gestational weeks 24–42.
Regional Notes: Birth weight reference ranges vary by population. In India, the average full-term birth weight is approximately 2,800–3,200 g. In the United States, the average is about 3,300–3,500 g (7.3–7.7 lb). In the UK, the average birth weight is approximately 3,300–3,400 g. These differences reflect variations in maternal nutrition, healthcare access, and ethnic demographics. Always consult your paediatrician or neonatologist for a complete growth assessment.
Frequently Asked Questions
What is birth weight percentile?
A birth weight percentile ranks a newborn's weight compared to other babies of the same gestational age and sex. The 50th percentile means half weigh more and half weigh less. Babies below the 10th percentile are classified as SGA (Small for Gestational Age), while those above the 90th percentile are classified as LGA (Large for Gestational Age), both of which may require monitoring.
What is the average birth weight?
The average birth weight for full-term babies (39–40 weeks) is approximately 3,300–3,500 g (7.3–7.7 lb). At 38 weeks, the average is about 3,100 g; at 40 weeks, 3,400 g; and at 42 weeks, 3,500 g. Baby boys are typically 100–150 g heavier than baby girls on average.
What does SGA mean?
SGA stands for Small for Gestational Age, meaning the baby's weight is below the 10th percentile for their gestational age. Causes include maternal hypertension, smoking, malnutrition, infection, and placental insufficiency. SGA infants need closer monitoring for hypoglycemia and other complications after birth.
What birth weight percentile is considered normal?
The 10th to 90th percentile range is considered Appropriate for Gestational Age (AGA), and about 80% of babies fall within this range. Some clinicians use the broader 5th to 95th percentile range. Percentiles alone do not determine overall health, and babies at the extremes can still be perfectly healthy.
How is gestational age calculated?
Gestational age is calculated from the first day of the last menstrual period (LMP) or estimated by early ultrasound. The due date is 40 weeks from LMP. Birth between 37 and 42 weeks is considered term, before 37 weeks is preterm, and after 42 weeks is post-term.
What factors affect birth weight?
Maternal nutrition is the most important factor. Smoking during pregnancy can reduce birth weight by approximately 200 g. Diabetes increases birth weight, while hypertension decreases it. Other factors include maternal weight gain during pregnancy, genetics, the baby's sex (boys are typically heavier), and gestational age at delivery.
Can birth weight predict future health?
Low birth weight is linked to a higher risk of hypertension, diabetes, and cardiovascular disease later in life. High birth weight (over 4,000 g) is associated with obesity and certain cancer risks. However, many other genetic and environmental factors matter more for long-term health outcomes.
Why do some newborns lose weight after birth?
It is normal for newborns to lose 7–10% of their birth weight in the first few days after delivery. This weight loss is primarily due to the excretion of meconium (first stool) and urine, as well as insensible fluid loss. Most babies regain their birth weight within 10–14 days with adequate feeding.