When parents look at a growth chart, they typically focus on one question: "What percentile is my child?" But pediatricians and growth specialists focus on a different, more powerful question: "How fast is my child growing between visits?" This is growth velocity — and it can detect problems that a single percentile reading completely misses.
Attained Growth vs Growth Velocity
Attained growth is a snapshot: your child's weight, height, or head circumference at a specific point in time, expressed as a percentile or z-score. It answers "where are they now?"
Growth velocity is a rate: how much your child has grown over a defined time interval. It answers "how fast are they growing?" and — crucially — "is their growth accelerating or decelerating?"
Clinical principle: A child consistently at the 5th percentile who maintains their channel is healthy. A child who drops from the 60th to the 30th percentile in 3 months needs investigation — even though both values are "within normal range."
How to Calculate Growth Velocity
Growth velocity is simply the change in measurement divided by the time interval:
Velocity = (Measurement₂ − Measurement₁) / Time interval
It is expressed as g/day (weight), cm/month (height), or cm/week for newborns. Example: if your 6-month-old weighed 6.0 kg and now weighs 6.9 kg at 9 months, that's 900g over 90 days = 10 g/day.
WHO Reference Velocity Values
| Age interval | Weight velocity (g/day) | Height velocity (cm/month) |
|---|---|---|
| 0–3 months | 25–35 | 3.5–4.0 |
| 3–6 months | 15–20 | 2.0–2.5 |
| 6–9 months | 10–14 | 1.5–2.0 |
| 9–12 months | 8–12 | 1.2–1.5 |
| 12–18 months | 6–8 | 1.0–1.2 |
| 18–24 months | 5–7 | 0.8–1.0 |
These are median values. Velocity below the 3rd percentile for age is a clinical signal requiring evaluation.
Growth Faltering vs Normal Variation
The term "failure to thrive" has largely been replaced by the more precise "growth faltering" — defined as weight-for-age z-score below −2, or a sustained downward crossing of percentile lines. Velocity data is critical here: a single low reading might be measurement error or a bad day; two consecutive velocity measurements below the 3rd percentile is a genuine signal.
The WHO-recommended approach is:
- Plot two measurements at least 4 weeks apart
- Calculate velocity in g/day or cm/month
- Compare to WHO velocity references for the age interval
- If velocity is below the 3rd percentile for the interval, investigate
Why Regular Measurements Matter So Much
You cannot calculate velocity from a single measurement. This is why the AAP recommends weight checks at well-child visits at 2 weeks, 2, 4, 6, 9, 12, 15, 18, and 24 months. Each new measurement gives you a velocity data point from the previous visit — and the trend over 3–4 visits gives an accurate picture of growth trajectory.
Frequently Asked Questions
What is a concerning weight velocity for a 6-month-old?
My child's percentile stayed the same but they grew slowly. Is that possible?
Does GrowChart track growth velocity?
See your child's growth velocity automatically
GrowChart calculates weight and height velocity between every pair of measurements and shows the trend alongside WHO reference ranges.