Baby height and length by age: how tall should my baby be?

There is no single correct length. Healthy babies vary widely, and the pattern over months matters far more than any one number. Most babies gain around 25 cm in the first year and about 12 cm in the second. Length is also the least reliable measurement taken on a baby, so single readings wobble.

How long should a baby be? The short answer

Most parents arrive at this question straight from an appointment, where a number was written into the personal child health record: the red book in the UK, the child health record book in Australia, the Rourke Baby Record sheet in Canada, the well child visit summary in the US. The number lands on a chart with curved lines on it, and a perfectly ordinary baby suddenly looks like a data point that needs explaining.

So here is the honest version. A percentile is a comparison with thousands of other babies of the same age and sex. It is not a score, not a grade, and not a ranking. A baby whose length sits on a lower line is not doing worse than a baby on a higher line, in exactly the same way that a shorter adult is not a worse adult. Somebody has to be on every line, including the low ones and the high ones, and almost all of those babies are perfectly well.

What clinicians actually watch is the shape of your baby's own line over time. A baby who tracks steadily along a low line, month after month, is usually far less interesting to a doctor than a baby who was tracking high and then flattened out. Direction beats position, nearly every time.

One naming note, because it confuses people: under the age of two we say length, not height, because the baby is measured lying down. Height is what we measure when a child can stand.

Average baby length by age

The table below comes from the WHO Child Growth Standards, which describe how healthy, breastfed babies grow in optimal conditions. These are the charts behind the UK-WHO charts in the red book, the charts the CDC recommends in the US for children under two, and the charts used in Canadian and Australian child health records. Your own book may print more lines than the three shown here, but the underlying data is the same.

Read across for your baby's age and down the column you are interested in. Roughly 94 out of every 100 babies measure somewhere between the lowest and highest columns shown, which should tell you how wide the healthy range genuinely is.

On a small screen, swipe the table sideways to see every column.

Figures come from the WHO Child Growth Standards, which apply to babies born at term; babies born preterm should be plotted using corrected age or on a chart recommended by their own doctor.

If the lines, numbers and jargon on the chart itself are the confusing part, our guide to what growth percentiles mean and how to read a growth chart walks through it properly, including why crossing a line is sometimes nothing and sometimes worth a mention.

How fast babies actually grow in length

Totals are only half the answer. What parents usually want alongside them is the rate: how much should this be changing between now and the next appointment? As broad averages, with plenty of healthy variation around them:

• Birth to 3 months: roughly 3 to 4 cm a month. This is the fastest your child will ever grow.
• 3 to 6 months: about 2 cm a month.
• 6 to 12 months: about 1 to 1.5 cm a month.
• The second year: roughly 1 cm a month.

Added up, that means most babies grow about 25 cm across the first year and about 12 cm across the second. Both figures are averages with wide healthy variation, so treat them as a shape rather than a target to hit.

Growth also is not smooth. Babies grow in bursts, with quiet stretches in between, and a burst can happen in a matter of days. Over a two week window you may see almost nothing, then a jump. This is normal and it is one reason short measuring intervals are so unhelpful: the closer together two measurements are, the more the noise drowns out the signal.

Why the length on the chart keeps jumping around

This is the part almost nobody explains, and it is the single most useful thing on this page. Length is the least reliable measurement taken on a baby, by a wide margin. Weight is easy: put the baby on the scale. Length is a genuine two person procedure done on a small human who has strong opinions about it.

Done properly, it needs a flat measuring board, one person holding the head steady against the fixed end while the baby looks straight up, and a second person straightening both legs gently, pressing the knees flat and bringing the sliding foot piece up against the soles with the feet at right angles. All of this happens while the baby curls up, arches, kicks, or screams. A tape measure alongside a wriggling baby on a changing mat is worse again, because a baby's natural position is curled, and a curled baby measures short.

The result is that errors of one to two centimetres are completely ordinary. Two experienced people measuring the same baby ten minutes apart can easily differ by a centimetre. That is not carelessness. It is the honest limit of the measurement.

Which brings us to the thing that frightens parents most: the chart showing that the baby got shorter. Babies do not shrink. It does not happen. If a length reading is lower than the previous one, the difference is in the measuring, not in the child. The sensible response is not a week of quiet panic and a spiral of late night searching, it is to ask for a repeat measurement.

A few things make a length measurement more trustworthy, and they are worth knowing so you can tell a solid number from a shaky one:

• Two people, not one. A single adult cannot hold the head and straighten the legs at the same time.
• A proper flat board with a fixed head end and a sliding foot piece, rather than a tape measure or marks on a mat.
• Head touching the fixed end, body straight, legs gently extended, knees pressed flat, feet flat against the foot piece.
• Hats, shoes and bulky clothing off, and the same approach used each time.
• Where possible, the same setting and the same measurer, so you are comparing like with like.
• Write down every reading, even the odd looking ones. Three points make a trend. One point makes nothing.

Lying down to standing up: the step at age two

Around the second birthday, the measurement itself changes. Up to two, your child is measured lying down as length. From two, once they can stand well, they are measured standing as height. Children measure slightly shorter standing than lying, because the spine settles under their own weight and the technique is different.

Growth charts are drawn with a small step at that changeover for exactly this reason. If your child's plotted point drops a little at the switch, that is the measurement changing, not your child losing anything. Comparing a lying measurement at two years with a standing measurement at two years and three months and concluding that growth stalled is comparing two different things.

Length, weight, body shape and family height

No clinician looks at length on its own. Length, weight and head circumference are read together, because it is the relationship between them that carries the information. A long, lean baby and a shorter, more solid baby can both be growing beautifully. Babies come in shapes, just as adults do.

Family height matters enormously here, and it is routinely underweighted by worried parents. A consistently short baby with two short parents, growing steadily along their own line, is usually just doing what the family does. The same is true in reverse for tall families. Genetics sets the channel; the first couple of years are partly your baby finding it.

That is also why length percentile in the first year does not reliably predict adult height. Plenty of babies drift up or down across lines during the first 18 to 24 months as they settle onto their own trajectory. Doing adult height arithmetic from a six month reading is not a calculation, it is a guess with decimal places.

When a doctor should look at growth in length

Growth charts are a screening tool. They flag patterns worth a closer look; they never make a diagnosis, and neither does an app or an article. Only your own clinician, seeing your whole child, can say anything about whether they are well.

Contact your doctor, health visitor, family physician or child health nurse if you notice any of the following:

• Growth in length that flattens over a period of months, so the line runs sideways while your child gets older.
• A drop downward through two major lines on the chart that stays down rather than correcting.
• Length falling away while weight holds up, or weight falling away while length holds.
• Growth in length that is clearly out of step with head circumference.
• A lower reading than last time that is still lower after a careful repeat measurement.
• Anything else that feels off: feeding, energy, alertness, wet nappies or diapers, or a baby who simply does not seem themselves.
• Your baby was born preterm and you are not sure whether the chart is using corrected age.

You do not have to wait for the next scheduled check. Bringing your own written record of the measurements you have collected makes that conversation considerably easier, because a trend answers questions that a single number never can.