Every parent finds themselves watching their baby closely — counting centimetres, checking weight at every visit, wondering whether their little one is growing the way they should. This guide walks you through exactly how babies grow in the first two years, what the UK growth charts mean, and what to do if you have any concerns.
How fast do babies grow?
In the first year of life, babies grow faster than at any other point outside the womb. Most full-term newborns arrive weighing between 6lb and 9lb (2.7–4.1kg) and measuring around 48–52cm in length. By their first birthday, the average baby has tripled their birth weight and grown around 25cm in height.
Growth is not a steady, straight line — it tends to happen in bursts. You may notice your baby feeding more than usual or sleeping longer for a few days, and then suddenly their vest no longer fits. These are growth spurts, and they are completely normal.
Baby growth chart: weight and length by age
The figures below are based on UK WHO growth charts and represent the average (50th centile) for a full-term baby. Your baby does not need to match these numbers exactly — what matters is that they follow a consistent pattern on their own centile line.
| Age | Average weight (girls) | Average weight (boys) | Average length |
|---|---|---|---|
| Birth | 3.2kg (7.1lb) | 3.4kg (7.5lb) | 49–51cm |
| 1 month | 4.2kg (9.3lb) | 4.5kg (9.9lb) | 53–55cm |
| 2 months | 5.1kg (11.2lb) | 5.6kg (12.3lb) | 56–59cm |
| 3 months | 5.8kg (12.8lb) | 6.4kg (14.1lb) | 59–62cm |
| 4 months | 6.4kg (14.1lb) | 7.0kg (15.4lb) | 62–65cm |
| 6 months | 7.3kg (16.1lb) | 7.9kg (17.4lb) | 65–68cm |
| 9 months | 8.2kg (18.1lb) | 9.0kg (19.8lb) | 70–73cm |
| 12 months | 9.2kg (20.3lb) | 10.0kg (22lb) | 74–77cm |
| 18 months | 10.5kg (23.1lb) | 11.1kg (24.5lb) | 80–83cm |
| 24 months | 11.5kg (25.4lb) | 12.3kg (27.1lb) | 85–88cm |
These are averages. A healthy baby can sit anywhere from the 2nd to the 98th centile and be developing perfectly well. Your health visitor will track your baby’s individual centile and flag any concerns.
Month-by-month growth milestones
0–3 months: The fastest growth period after birth. Most babies regain their birth weight within 10–14 days and then gain roughly 150–200g per week. Head circumference also grows rapidly in these early weeks as the brain develops.
3–6 months: Growth continues at a strong pace, typically around 100–150g per week. By four months, many babies have doubled their birth weight. You will start to see the baby rolls that signal good feeding and healthy fat stores.
6–9 months: Weight gain begins to slow slightly as babies become more mobile. Many start solids around six months, which introduces a new energy source alongside milk. Expect around 70–90g per week during this stage.
9–12 months: Growth slows further as activity increases — crawling and pulling to stand burns more energy. Babies typically gain around 50–70g per week in the final quarter of the first year.
12–24 months: The growth rate continues to settle. Toddlers may seem to eat less than you expect, but this is normal as the extraordinary pace of the first year eases.
What affects baby growth rate?
Several factors influence how quickly your baby grows, and most are perfectly natural:
- Genetics — height and build run in families. A baby with two tall parents is likely to track on a higher centile than a baby with two shorter parents.
- Feeding — whether breastfed or formula-fed, adequate milk intake is the primary driver of growth in the first six months. Breastfed babies often gain weight slightly differently from formula-fed babies, particularly in the early months.
- Prematurity — babies born early use corrected age (adjusted for how many weeks early they arrived) on growth charts until the age of two. If your baby was premature, your health visitor will use their corrected age, not their actual birthday, as the reference point.
- Illness — a cold, a bout of sickness, or teething can temporarily slow weight gain. Babies usually catch up within a week or two once they recover.
- Sleep — most growth hormone is released during deep sleep, so longer stretches of sleep often correspond with noticeable growth spurts.
Growth spurts: what to expect
Growth spurts tend to cluster around the same ages for most babies: around 2–3 weeks, 6 weeks, 3 months, 6 months, and 9 months. During a spurt, your baby may:
- Feed more frequently or seem hungrier than usual
- Be fussier or harder to settle
- Sleep more or, confusingly, sleep less well
- Wake more at night even if they had been sleeping through
Growth spurts usually last just a few days. Following your baby’s hunger cues — feeding on demand rather than by the clock — is the best way to support them through it.
When to speak to your health visitor
Growth charts are a guide, not a rule. That said, it is worth speaking to your health visitor if:
- Your baby’s weight crosses two centile lines downward over a period of time
- Your baby is not regaining birth weight by two weeks old
- You are concerned about feeding — too little, or difficulty latching
- Your baby seems unwell, unusually lethargic, or is having noticeably fewer wet nappies
In most cases there is a straightforward explanation, but early conversations with your health visitor are always the right call if something feels off.
Frequently asked questions
How much should a newborn gain per week?
Most newborns lose a small amount of weight in the first few days (up to 10% of birth weight) and then begin gaining. From around day 4–5, expect roughly 150–200g per week for the first three months.
Do breastfed and formula-fed babies grow at the same rate?
In the early weeks, formula-fed babies can gain weight slightly faster. By six months the patterns even out. The UK uses WHO growth charts based on breastfed babies as the reference, so both groups are compared to the same standard.
My baby dropped a centile — should I be worried?
A single drop is not usually a cause for alarm, particularly around the time solids are introduced or during an illness. A sustained downward drift across two centile lines is worth discussing with your health visitor.
When do babies have their biggest growth spurts?
The most dramatic growth happens in the first three months. After that, growth continues steadily but slows progressively through the first and second year.
How do I know what clothing size my baby needs?
UK baby clothing sizes are typically based on age, but weight is a more reliable guide — especially in the early months. If your baby is on a higher or lower centile for weight, always size by weight rather than age.
As your baby grows through each stage, find soft, beautiful pieces sized to fit in our baby clothing collection — or explore our dedicated premature range for the very smallest sizes.