What is a normal weight percentile for a baby?

Any percentile from 2nd to 98th is considered normal. A baby on the 25th percentile is just as healthy as one on the 75th. What matters most is consistent growth along their own curve, not which percentile they're on.

Should I worry if my baby is on a low percentile?

Not necessarily. The 9th percentile means 9% of babies weigh less - that's normal for some children. Concern arises if a baby drops across two or more percentile lines, or falls below the 0.4th percentile. Consistent growth on any line is usually healthy.

When should I be concerned about my baby's weight?

Speak to your health visitor or GP if: weight crosses downward through two percentile lines, weight is below the 0.4th percentile, there's sudden weight loss, or the baby shows signs of illness alongside weight changes. Single measurements are less important than the overall trend.

Baby Weight Percentile Calculator Guide

Growth charts and percentiles can seem confusing for new parents, but they're simply tools to track your baby's development over time. This guide explains what percentiles mean, how UK growth charts work, and when you might need to speak to a health professional.

What Are Percentiles?

A percentile shows where your baby's weight falls compared to other babies of the same age and sex. If your baby is on the 50th percentile, they weigh more than 50% of babies and less than 50% of babies their age.

91st

Heavier than 91% of babies

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50th

Right in the middle

25th

Heavier than 25% of babies

9th

Heavier than 9% of babies

Key Point: A baby on the 9th percentile is just as healthy as one on the 91st percentile. What matters is that they follow their own growth curve consistently, not which percentile line they're on.

UK-WHO Growth Charts

The UK uses growth charts based on World Health Organization (WHO) data, showing how breastfed babies grow under optimal conditions. These charts are in your child's Red Book (Personal Child Health Record).

Percentile Lines on UK Charts

Percentile	Meaning	Approx. % of babies above
99.6th	Very high	0.4%
91st	Above average	9%
75th	Upper average	25%
50th	Average	50%
25th	Lower average	75%
9th	Below average	91%
0.4th	Very low	99.6%

Average Baby Weights (UK)

Boys

Age	9th Percentile	50th Percentile	91st Percentile
Birth	2.8 kg	3.5 kg	4.2 kg
3 months	5.1 kg	6.4 kg	7.7 kg
6 months	6.6 kg	8.0 kg	9.6 kg
9 months	7.6 kg	9.2 kg	10.9 kg
12 months	8.4 kg	10.1 kg	11.9 kg
18 months	9.5 kg	11.5 kg	13.6 kg
2 years	10.4 kg	12.5 kg	15.1 kg

Girls

Age	9th Percentile	50th Percentile	91st Percentile
Birth	2.6 kg	3.3 kg	4.0 kg
3 months	4.6 kg	5.8 kg	7.1 kg
6 months	6.0 kg	7.4 kg	9.0 kg
9 months	7.0 kg	8.5 kg	10.4 kg
12 months	7.7 kg	9.4 kg	11.3 kg
18 months	8.8 kg	10.8 kg	13.0 kg
2 years	9.8 kg	12.0 kg	14.6 kg

When to Be Concerned

Speak to your health visitor or GP if:

Weight crosses downward through two or more percentile lines
Weight is below the 0.4th percentile
Weight is above the 99.6th percentile
There's a sudden change in growth pattern
You have concerns about feeding or your baby seems unwell

Faltering Growth: If your baby's weight drops through two percentile lines (e.g., from 50th to 9th), this is called "faltering growth" and should be assessed by a healthcare professional. It doesn't necessarily mean something is wrong, but it needs investigating.

Normal Growth Patterns

Some variation is completely normal:

First Two Weeks

Babies typically lose 5-10% of birth weight in the first few days
Most regain birth weight by 10-14 days
Weight loss over 10% needs medical attention

First Year

Rapid growth in first 3-4 months
Growth rate slows from 4-12 months
Birth weight typically doubles by 4-5 months
Birth weight typically triples by 12 months

After One Year

Growth slows significantly
Toddlers may gain only 1-2 kg per year
Appetite often decreases—this is normal

Growth Spurts: Babies often have growth spurts around 2-3 weeks, 6 weeks, 3 months, and 6 months. During these times, they may feed more frequently and seem hungrier than usual.

Factors Affecting Weight

Many factors influence where your baby falls on the growth chart:

Genetics: Larger parents often have larger babies
Birth weight: Premature or low birth weight babies may track lower
Feeding method: Breastfed and formula-fed babies grow differently
Sex: Boys are typically slightly heavier than girls
Ethnicity: Growth patterns can vary by ethnic background
Illness: Temporary illness can affect weight

Example: Catching Down

A baby born on the 91st percentile to average-sized parents may naturally "catch down" to the 50th percentile in the first months. This is normal genetic adjustment, not a concern.

Example: Catching Up

A premature baby born on the 9th percentile may "catch up" to the 50th percentile as they grow. Premature babies are measured on corrected age (from their due date, not birth date) until age 2.

Length/Height and Head Circumference

Weight isn't the only measurement tracked:

Length (measured lying down until age 2)

Average birth length: 50 cm
Grows about 25 cm in first year
Grows about 12 cm in second year

Head Circumference

Average at birth: 34-35 cm
Grows about 12 cm in first year
Tracks brain growth and development

Proportional Growth: Healthcare professionals look at all measurements together. A baby on the 25th percentile for both weight and length is proportionally small and perfectly healthy. Concern arises when measurements are very different (e.g., 91st for weight, 9th for length).

Red Book Measurements

In the UK, your baby will be weighed:

At birth
First two weeks: To check they regain birth weight
6-8 week check: GP appointment
Health visitor reviews: Usually around 9-12 months and 2-2.5 years
On request: At baby clinics if you have concerns

Weighing Frequency: Weighing too often can cause unnecessary worry. After the first few weeks, monthly weighing is usually sufficient. For healthy babies, weighing every 2-3 months is often enough.

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Summary

Percentiles compare your baby to others of the same age and sex
Any percentile from 0.4th to 99.6th can be normal
Consistent growth along any curve is more important than the percentile number
Crossing two or more lines warrants a check-up
Your health visitor is there to help with any concerns

Baby Health Monitoring in the UK: NHS Guidelines and Support

The United Kingdom provides one of the most comprehensive child health monitoring programmes in the world through the NHS Healthy Child Programme. Every baby born in the UK receives a Personal Child Health Record (PCHR), commonly known as the Red Book, at birth. This book is used by parents, health visitors, and GPs to track a child's growth, development, and immunisations from birth through to age five. Weight measurements are plotted on UK-WHO growth charts, which were adopted by the NHS in 2009 to replace the previous charts based on formula-fed babies.

Health visitors, who are specialist community public health nurses employed by the NHS, play a vital role in supporting families across England, Scotland, Wales, and Northern Ireland. They conduct a series of mandated health reviews, including the new baby review (within 10 to 14 days of birth), the 6-to-8-week review, the 1-year review, and the 2-to-2.5-year review. During these visits, your baby will be weighed and measured, and the results plotted on their growth chart. Health visitors can provide personalised advice on feeding, sleep, and development based on your baby's individual growth pattern.

If you have concerns about your baby's weight between scheduled reviews, you can visit an NHS baby clinic, which most GP surgeries and children's centres offer weekly. These drop-in clinics are free and staffed by health visitors or nursery nurses who can weigh your baby and provide reassurance or referral if needed. The NHS Start4Life programme also offers free resources, including a dedicated helpline and online guides covering breastfeeding, weaning, and toddler nutrition tailored specifically for UK families.

For premature babies, the NHS uses corrected age (adjusted for how early the baby was born) when plotting growth charts until the child reaches two years of age. This ensures that premature babies are assessed fairly against appropriate developmental milestones. The Bliss charity, the UK's leading organisation for premature and sick babies, provides extensive support and information for parents navigating the NICU experience and beyond.

Frequently Asked Questions About Baby Weight Percentiles

What growth charts does the NHS use for babies in the UK?

The NHS uses UK-WHO growth charts, which were introduced in 2009. These charts are based on data from the World Health Organization's Multicentre Growth Reference Study, which tracked the growth of breastfed babies across six countries. For babies from birth to age one, the charts reflect the growth of exclusively or predominantly breastfed infants. From age one to four, WHO data is used, and from age four onwards, UK 1990 reference data is applied. These charts are printed in every Red Book and are also available digitally through the Royal College of Paediatrics and Child Health (RCPCH) website.

How often should my baby be weighed according to NHS guidelines?

According to the NHS Healthy Child Programme, babies should be weighed at birth, during the new baby review (10 to 14 days), and at the 6-to-8-week check. After that, the NHS recommends weighing no more than once a month up to age six months, once every two months from six to twelve months, and once every three months after their first birthday. Weighing too frequently can cause unnecessary anxiety, as normal daily fluctuations in hydration and feeding can make it appear that a baby's growth is erratic when it is actually perfectly healthy.

Should I be worried if my baby drops a percentile line on their growth chart?

A drop across one percentile line is usually not a cause for concern and can happen during normal growth. The NHS advises that health professionals should investigate further if a baby drops across two or more percentile lines, as this may indicate an underlying issue such as feeding difficulties, illness, or food intolerance. However, some healthy babies naturally adjust their growth trajectory in the first year of life, particularly if they were born larger or smaller than average. Your health visitor will consider the overall pattern of growth rather than any single measurement and can refer you to a paediatrician if necessary.

UK-WHO Growth Charts and Health Visitor Monitoring

In the United Kingdom, child growth monitoring follows the UK-WHO growth charts, which were adopted by the Department of Health in 2009 to replace the earlier 1990 UK reference charts. These charts are based on data from the World Health Organization's Multicentre Growth Reference Study, which tracked the growth of breastfed infants from six countries under optimal conditions. The UK-WHO charts are printed in every child's Personal Child Health Record (commonly known as the Red Book), which parents receive shortly after birth. Health visitors use these charts at routine developmental reviews, typically at one to two weeks, six to eight weeks, and then at nine to twelve months, to plot a child's weight, length, and head circumference against national percentile standards.

Health visitors play a central role in the UK's child health surveillance programme, which is coordinated by local NHS trusts. The Healthy Child Programme, published by Public Health England, sets out the schedule of universal health reviews that every child in England is entitled to receive. In Scotland, the Universal Health Visiting Pathway provides an even more intensive schedule, with eleven home visits offered in the first year of life. These visits are not simply about measuring weight; they encompass feeding support, developmental assessments, maternal mental health screening, and safeguarding observations. If a health visitor identifies concerns about a child's growth trajectory, they can refer directly to paediatric services, dietitians, or speech and language therapists without requiring a GP appointment, ensuring that early intervention reaches families as quickly as possible.

Infant Feeding Support and Weight Monitoring Across the UK

Infant feeding practices are closely linked to weight percentile tracking in the United Kingdom, and the NHS provides extensive support to help parents establish successful feeding, whether through breastfeeding, formula feeding, or a combination of both. The UK has set ambitious targets to increase breastfeeding rates, with Public Health England and the devolved health services in Scotland, Wales, and Northern Ireland all implementing the UNICEF Baby Friendly Initiative across maternity units and community health services. This evidence-based programme trains healthcare professionals to support mothers with breastfeeding initiation and continuation, and hospitals that achieve Baby Friendly accreditation have demonstrated higher breastfeeding rates at discharge and at six to eight weeks postnatal.

For parents who are concerned about their baby's position on the weight percentile chart, the NHS Start4Life programme offers free, evidence-based guidance on infant nutrition, including detailed information on introducing solid foods at around six months of age, as recommended by the Department of Health and Social Care. The National Child Measurement Programme (NCMP), which measures the height and weight of children in Reception (aged four to five) and Year 6 (aged ten to eleven) in England, provides population-level data on childhood weight trends and helps identify areas where additional public health intervention may be needed. Parents in the UK can access free support from their health visitor, GP, or local children's centre if they have concerns about their child's weight or feeding, and the NHS website provides comprehensive resources covering every stage of infant and child nutrition from birth through to school age and beyond.

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Dr. Sarah Chen, PhD

Research Scientist, Public Health

Sarah holds a PhD in Public Health from the University of Edinburgh and has published research on UK health metrics and obesity trends. She translates complex medical data into practical, accessible guidance for everyday readers.