Is your baby's weight normal for their age?

A modest percentile shift is common and usually not concerning. Babies naturally settle into their genetic growth channel over the first 12-18 months, which can involve crossing percentile lines. A drop of 2 or more major centile lines (e.g., from 75th to 25th) is more likely to warrant discussion with your health visitor. A single reading is less informative than the trend across multiple visits.

Use your baby's corrected age (chronological age minus weeks premature) until at least 24 months. A baby born at 32 weeks (8 weeks early) who is now 4 months old chronologically has a corrected age of approximately 2 months. This calculator applies corrected age automatically when you enter gestational age.

Any percentile from the 3rd to the 97th is considered within the normal range on the WHO growth chart. A baby at the 10th percentile is simply lighter than average but perfectly healthy. A baby at the 90th percentile is heavier than average and equally healthy. What matters is consistent growth along the baby's own percentile channel over time, not the specific number at any single visit.

Birth weight has only a weak correlation with adult body weight. The main factors driving adult weight are genetics (height and body composition), nutrition, and physical activity established during childhood and adolescence. Babies who are large at birth are not destined to be overweight adults, and babies who are small at birth are not destined to be lean adults. The relationship between birth weight and long-term health outcomes is more relevant for very low birth weight infants (under 1.5 kg), where catch-up growth patterns have been associated with increased metabolic risk in adulthood.

Different countries use different reference charts. The UK uses the UK-WHO growth charts, which are an adaptation of the WHO 2006 standards with UK-specific data blended in for older age ranges. The US Centres for Disease Control and Prevention (CDC) has its own growth charts based on US population data, though the AAP recommends using the WHO charts for children under 2. This calculator uses the international WHO Child Growth Standards, which are widely regarded as the best reference for optimal infant growth regardless of ethnicity.

The WHO Child Growth Standards (2006) were developed from the Multicentre Growth Reference Study, which enrolled breastfed children from six countries growing in optimal conditions. They represent how children should grow under ideal circumstances. The CDC charts (2000) are based on US population data and describe how US children have grown historically, which includes a higher proportion of formula-fed infants. For children under 2, the AAP recommends the WHO charts; for ages 2 and over, the CDC charts are more commonly used in the United States.

Concern is warranted when a baby crosses 2 or more major centile lines downward over time (for example, dropping from around the 75th to the 25th), refuses feeds consistently, fails to regain birth weight by 2 weeks, or shows other signs such as lethargy, poor nappy output, or pallor. A single measurement below or above average is not a cause for concern. It is the trend across multiple measurements, ideally taken at consistent intervals by the same healthcare provider using the same equipment, that gives a meaningful picture of growth.

Most newborns lose 5 to 10% of their birth weight in the first few days of life as they lose fluid and establish feeding. This is normal and expected. Babies typically regain their birth weight by 10 to 14 days of age. Weight loss exceeding 10% warrants assessment, and loss exceeding 7% before day 3 may prompt a feeding review. After regaining birth weight, typical gains are 150 to 200 grams per week in the first 3 months, slowing to around 100 grams per week between 3 and 6 months.