Average weight gain by month, what percentile lines mean, and how to read your baby's WHO growth chart without panicking.
Your baby is in the 12th percentile. Or the 87th. Or a different one each month. You're not sure whether to be worried, proud, or both.
Percentile lines confuse almost every parent at first. Once you understand what they actually represent, the chart becomes one of the most useful tools you have.
Average weight gain by age
Averages first, percentile context after. These are typical weekly gains for a healthy baby on the 50th percentile:
| Age | Weekly gain | Approx. monthly gain |
|---|---|---|
| 0–3 months | 150–200 g (5–7 oz) | 700–900 g (1.5–2 lb) |
| 3–6 months | 100–150 g (3.5–5 oz) | 450–650 g (1–1.4 lb) |
| 6–12 months | 50–100 g (2–3.5 oz) | 200–400 g (0.5–0.9 lb) |
| 12–24 months | ~25 g (~1 oz) | ~110 g (~0.25 lb) |
| 2–5 years | ~2 kg/year (~4.4 lb/yr) | – |
Important: weekly variation is huge. Don't judge growth on a single weigh-in. The trend over 4 weeks is the real signal.1
Average weight by age
These are very rough WHO 50th-percentile figures (boys and girls combined for simplicity — actual charts split by sex):
| Age | Weight (kg) | Weight (lb) |
|---|---|---|
| Birth | 3.3 | 7.3 |
| 1 month | 4.4 | 9.7 |
| 3 months | 6.0 | 13.2 |
| 6 months | 7.6 | 16.8 |
| 9 months | 8.6 | 19.0 |
| 12 months | 9.4 | 20.7 |
| 18 months | 10.7 | 23.6 |
| 24 months | 11.8 | 26.0 |
Half of healthy babies are above these numbers and half below. Being away from the average isn't a problem on its own.
What percentile lines actually mean
A growth chart plots your baby's weight against the WHO Child Growth Standards — data from healthy, breastfed babies of all body types worldwide.2
A percentile is just a ranking:
- 50th percentile = average. Half of babies weigh more, half weigh less.
- 10th percentile = your baby is heavier than 10% of babies their age.
- 90th percentile = your baby is heavier than 90% of babies their age.
Most babies aren't at the 50th, because by definition only some of them can be. A 10th-percentile baby and a 90th-percentile baby can both be perfectly healthy.
The percentile is descriptive, not a target. A baby who is consistently on the 15th percentile and growing along that line is doing exactly what they should. A baby who has dropped from the 75th to the 25th is the one to look at, even if they're "above average."
Tracking the line, not the number
The single most useful thing on the chart is the line your baby draws across it over time. Healthy babies tend to settle into a percentile band by around 6 months and stay roughly on it.
Two patterns matter:
- Crossing 2+ percentile bands downward — falling from the 75th to the 25th, for example. This is the textbook concerning pattern, and is what pediatricians actually watch for.
- Crossing 2+ percentile bands upward — usually less worrying, but worth mentioning to your pediatrician, especially in older babies on solids.
A baby zig-zagging by one band each month is normal noise.
Why your baby's percentile might shift
Common, mostly harmless reasons percentiles move:
- Big birth weight, smaller frame catching up. Many babies born on the 90th percentile slowly settle to a lower band over the first 6 months as their body composition rebalances.
- Small birth weight, catch-up growth. The reverse — small babies often climb percentiles in the first 6 months.
- Solids transition. Once solids start, growth often plateaus briefly while the gut adjusts. Expect dips around 6–9 months.
- Activity surge. Once babies start crawling and walking, they often lose visible chub even while gaining weight slowly. The chart sometimes flattens here.
WHO vs CDC charts
If you've seen two different charts, that might be why. The WHO chart is the international standard for babies up to 24 months and reflects optimal growth in mostly breastfed populations. The CDC chart in the US is sometimes used for older children and is based on a more mixed-feeding population. The WHO chart is preferred under 2 years.2
When to call your pediatrician
Mention growth at your routine check-ups; call sooner if:
- Your baby has crossed 2+ percentile bands downward
- Weight has stalled for 4+ weeks
- Weight is dropping in absolute terms (not just slowing)
- Your baby has lost weight without an illness explaining it
- Slow growth alongside missed milestones, low energy, or feeding difficulty
For a single low or unusual weigh-in with a baby who's otherwise thriving, the answer is almost always: plot it, watch the trend, recheck in 2–4 weeks.
Plot every weigh-in
Weights are useful as a curve, useless as one-off numbers. Logging each weigh-in and seeing it plotted against the WHO chart shows you the actual trajectory — and gives your pediatrician a clearer record than your memory ever will.
← Back to the complete guide: Tracking baby weight
Also in this cluster: Newborn weight loss after birth · When does baby double birth weight · Slow weight gain in breastfed babies
Sources
- NHS. "Your baby's weight and height." NHS.uk, 2024. https://www.nhs.uk/conditions/baby/babys-development/height-weight-and-reviews/baby-height-and-weight/
- World Health Organization. "WHO Child Growth Standards." 2023. https://www.who.int/tools/child-growth-standards
- American Academy of Pediatrics. "Your Baby's Growth." HealthyChildren.org, 2023. https://www.healthychildren.org/English/ages-stages/baby/Pages/default.aspx
Footnotes
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NHS. "Your baby's weight and height." NHS.uk, 2024. https://www.nhs.uk/conditions/baby/babys-development/height-weight-and-reviews/baby-height-and-weight/ ↩
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World Health Organization. "WHO Child Growth Standards." 2023. https://www.who.int/tools/child-growth-standards ↩ ↩2