My child seems shorter than classmates. How can I check?

If your child looks smaller or shorter than their friends, measure their height properly, plot it on the growth chart in their health record and measure again from time to time, for example at check-ups, then look at the line over months to years. One measurement says little; the line the measurements make over time says much more. Being shorter than classmates is often normal: MedlinePlus, from the US National Library of Medicine, says some children may be small for their age but still be developing normally, and some are short because their parents are. Children in the same class can be almost a year apart in age and start puberty at different times. If growth seems very slow or the height is below the lowest line on the chart, talk to a doctor.

Is a child who is shorter than classmates usually growing normally?

Often, yes. Comparing your child with the classmates around them is natural, but it mixes up several things that have nothing to do with how well your child is growing.

Why classmates differ in height

When Thai children start puberty

Two studies from Ramathibodi Hospital, Mahidol University, assessed healthy Bangkok children in 1997 to 1999.

The study in boys shows how wide the normal spread is: the first signs of puberty appeared anywhere from 9.5 to 12 years. Both studies were done in Bangkok more than 25 years ago, so today's ages may be somewhat different, but the point stands: two healthy children of the same age can be at very different stages.

How do I measure my child's height correctly at home?

A careful measurement is the starting point, because a difference of 1 or 2 cm can come from how you measure rather than from growth.

Steps from the US CDC

The US CDC's guide to measuring children advises:

• remove shoes, bulky clothing, hats and hair ornaments
• stand the child on a hard floor such as tile or wood, not carpet, against a flat wall
• feet flat and together, legs straight, arms at the sides and shoulders level
• head, shoulders, buttocks and heels touching the wall
• lower a flat ruler or book until it firmly touches the top of the head, at a right angle to the wall
• read the mark with your eyes at the same level as the ruler, and record to the nearest 0.1 cm

Make it a routine

Measure at the same time of day, in the same place, and write the date and the height in your child's health record book. A series of measurements over time tells you much more than any single one.

How do I read a growth chart?

A growth chart shows where a child's height sits compared with many children of the same age and sex. The US CDC describes clinical growth charts as percentile curves: a percentile shows what percentage of the reference group a child equals or exceeds.

What the lines mean

The line over time matters more than one point

CDC guidance for health workers on using the WHO growth charts says growth monitoring is based on a series of accurate measurements over time. That guidance is written for charts used under age 2, but the same idea applies to older children. The same guidance says that when growth looks unusual, measurements are read together with the child's medical and family history, so a doctor reads the chart alongside what they know about your child.

Which chart is used can change the result

Thailand has its own national growth charts, and international WHO standards are also used. A study by Hong and colleagues used data on 4,224 children from the Prospective Cohort Study of Thai Children, from birth to 24 months. The Thai charts reflected Thai children better, while the WHO standards classified more children as short for their age. If you compare results from different clinics, check that they use the same chart.

How much do the parents' heights matter?

A lot. MedlinePlus Genetics says scientists estimate that about 80% of a person's height is determined by DNA variations inherited from their parents.

A rough guide from the parents' heights

Zeevi and colleagues describe the standard method doctors have used for decades: take the average of both parents' heights, add 6.5 cm for a son or subtract 6.5 cm for a daughter, and expect the child's adult height to fall within about 8.5 cm either side. The same paper notes that later guides used about 9 cm for girls and 10 cm for boys, and that the normal range has not yet been firmly established.

The same study, in 23 very large families in Israel and the United States, found that this method explained only 36% of the differences in the children's heights, and that the children were on average 2.7 cm taller than predicted. It is a rough guide, not a forecast.

Measure the parents too

A study by Braziuniene and colleagues at a children's clinic in the United States measured 98 fathers and 217 mothers and compared the results with the heights they reported. Only 50% of fathers and 58% of mothers reported their height within 2 cm of the measured value, and fathers tended to overstate theirs (174.9 cm reported against 173.2 cm measured on average). If a doctor asks about your heights, measure them rather than guess.

How do Thai surveys count children as short for their age, and how many are?

National surveys measure the height of young children across Thailand. They count a child as short for their age when their height is more than 2 standard deviations below the reference median for their age and sex, roughly below the lowest line on the chart.

What the national surveys found

The figures come from an analysis by Cetthakrikul and colleagues at the International Health Policy Program, Ministry of Public Health, who found that children in poorer households were more likely to be short for their age. These are figures for children under 5 measured once each, so they do not tell you about your own child. For that, the line on your child's chart over time is what counts.

How much sleep does a child need?

Sleep is part of every child's daily routine. The US CDC recommends the following, as general guidance for all children. It is not guidance about height:

Recommended sleep by age

Fitting it into a Thai school day

School mornings often start early, especially with long commutes in the city. An earlier bedtime on school nights, with screens put away beforehand, is usually the easiest place to find more sleep.

When should I take my child to a doctor about their height?

Talk to a doctor or a child health clinic whenever something about your child's growth worries you. MedlinePlus notes that very slow or very fast growth can sometimes signal a problem that needs checking.

Signs worth a visit

• your child seems to be growing very slowly
• your child is much shorter than the parents' heights would suggest
• your child is below the lowest printed line on the chart
• anything else about your child's growth or development worries you

What to bring

• the child's health record book with the dated measurements
• both parents' measured heights
• notes on any regular medicines, long illnesses or changes you have noticed

Before giving your child any medicine or product because of their height, ask the doctor.