On the UK reference chart, the average full-term baby weighs about 3.5 kg (7 lb 11 oz) for boys and 3.4 kg (7 lb 7 oz) for girls at 40 weeks. Most healthy term babies weigh between about 2.5 kg and 4.5 kg (5 lb 8 oz to 9 lb 15 oz). Low birth weight is under 2.5 kg (WHO definition).
Birth weight centiles at 40 weeks
| Centile | Boys | Girls |
|---|---|---|
| 0.4th | 2.18 kg (4 lb 13 oz) | 2.11 kg (4 lb 10 oz) |
| 2nd | 2.47 kg (5 lb 7 oz) | 2.39 kg (5 lb 4 oz) |
| 9th | 2.83 kg (6 lb 4 oz) | 2.73 kg (6 lb 0 oz) |
| 50th | 3.50 kg (7 lb 11 oz) | 3.36 kg (7 lb 7 oz) |
| 91st | 4.17 kg (9 lb 3 oz) | 3.99 kg (8 lb 13 oz) |
| 98th | 4.53 kg (10 lb 0 oz) | 4.33 kg (9 lb 9 oz) |
| 99.6th | 4.82 kg (10 lb 10 oz) | 4.61 kg (10 lb 3 oz) |
UK 1990 reference for babies born at 40 weeks. Babies born earlier or later are compared at their gestational age.
Low and high birth weight
Low birth weight
Under 2.5 kg (5 lb 8 oz). Often linked with being born early, twins, smoking in pregnancy or problems with the placenta. These babies may need extra feeding support and checks.
Large for gestational age
Above the 90th centile – often over about 4–4.5 kg at term. More common with diabetes in pregnancy, higher maternal weight or tall parents.
What affects birth weight?
- How many weeks pregnant you are when the baby is born.
- Sex – boys are on average slightly heavier.
- Parents’ size and family history.
- Smoking, high blood pressure or pre-eclampsia (lower weight).
- Diabetes in pregnancy (higher weight).
- Twins or more (lower weight).
After birth
Babies usually lose some weight in the first few days and regain their birth weight by about 2 weeks. Track their growth with the baby weight percentile calculator.
Frequently asked questions
What is the average baby weight at birth in the UK?
About 3.5 kg (7 lb 11 oz) for boys and 3.4 kg (7 lb 7 oz) for girls at full term.
Is a 6 lb baby small?
A 6 lb (2.7 kg) term baby is on roughly the 9th centile – smaller than average but within the normal range.
Is a 9 lb baby big?
A 9 lb (4.1 kg) term baby is around the 90th centile for boys and above it for girls – larger than average but common.
What is low birth weight?
Under 2.5 kg (5 lb 8 oz), according to the World Health Organization.
Related tools and guides
About this page
How this guide works
Birth centiles come from the UK 1990 reference for term babies (40 weeks), as used on the UK-WHO birth chart.
Sources
- NHS – Your baby’s weight and height
- RCPCH – UK-WHO growth charts
- WHO – Child growth standards
- World Health Organization. Low birth weight: definition (<2,500 g).
Medical disclaimer: this guide is general information for adults and is not a diagnosis or medical advice. bmicalculatornhs.net is independent and not affiliated with the NHS. Speak to your GP, pharmacist or another qualified health professional about your own health. Full disclaimer.