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What is "BMI"?

Adolescent Weight And Growth

Shannon Duffy (Registered Dietician & Child Nutritionist, Children's Hospital Los Angeles) gives expert video advice on: How do I calculate my child's BMI?; What is the BMI range for an underweight, normal and overweight child?; How do I encourage my child to maintain a healthy rate of growth and weight? and more...

What is "BMI"?

BMI is body mass index. Body mass index the measurement that we use, or a calculation that we use to determine whether your weight is appropriate for your height. In children, we then plot the BMI on a growth chart. In adults we look at that particular BMI number.

What is a "BMI percentile"?

A BMI percentile is a clinical indicator that we use to determine how your child compares to other children of the same age. For example, your BMI may be at the 50th percentile. That means that 50 percent of kids of your child's gender and your child's age will have a BMI less than your child, and 50 percent will have a BMI greater than your child.

Is BMI calculated the same way for children as it is for adults?

It's the exact same formula to obtain the BMI; what's different is in adults we just look only at the answer the formula produces, only at BMI. In kids, we take that BMI number, that answer the formula produces, and plot it on a growth chart based on their age and their gender.

What is the BMI range for an underweight, normal and overweight child?

The BMI range for children 2-20 years old is, on the growth chart, the third to the ninety-seventh percentile. If your child is less than the third percentile, than your child is underweight and malnourished. If your child's BMI for age is greater than the eighty-fifth percentile, your child is overweight. And if your child is greater than the ninety-fifth percentile, your child is at high risk of adult obesity.

What is a "growth chart"?

A growth chart is an indicator that we use to watch how quickly a child grows. The growth chart has been developed by the Centre for Disease Control. They conducted a study where they looked at all kids in America and evaluated how they grew. From that they were able to establish a growth chart showing a curve that coincides with how fast children should grow, how fast they should gain weight.

Are all children measured against the same growth chart?

There are different growth charts and in America we use growth charts developed by the CDC. There are various charts for different ages and each gender has their own growth curve. In the big wide world, the World Health Organization has been developing new growth charts that actually take in to account children all over the world. They are looking at how every child grows, so it is an international document. Right now, in America we only use the CDC charts and that's applicable only to childrens in America. Kids in Africa, or South America, or China, will probably have different growth rates and they may even have their own growth chart for that particular country.

Can I use a growth chart to predict my child's future height and weight?

Probably the simplest way to predict your child's height later on in life is to see where they are on their growth chart of height against age. You can assume, hopefully, that the kid's going to grow as they should. You can follow that curve until they're 18 years old, and you can estimate and have a guess at their eventual height. Otherwise, there's different formulas you can use to figure out future height, and your doctor will have those formulas.

What is the "growth percentile curve"?

The growth percentile curve is a line on the actual growth chart that indicates how a child should grow. That's our reference point on the growth chart. These are the growth percentile curves, and when we plot your child, we will find their age on the bottom. If they are seven years old and fifty two inches, for example, we'll follow the fifty-two inches along to the seven year old line, and if that is your child's height, then your child is between the ninetieth and ninety-seventh percentile. That tells me that 90% of boys who are seven years old have a height less than your child. 10% will have a height greater than your child. What's really important with growth charts is plotting over time and seeing how that child grows.

Should I be concerned if my child's growth percentile changes?

You definitely should be concerned if your child's height or weight changes off of the growth percentile curve that they've been on. For example, your child starts off at the 50th percentile for weight for age, meaning 50% of kids your child's age and gender weigh less than your child and 50% percent will weigh more. They may be growing following this 50th percentile growth curve, and then reach 11 years old and their growth starts to level off. Children's growth is not supposed to level out; their weight gain is not supposed to level out when they're 11 years old. That tells me that something is wrong. Vice-versa is also true. If your child is growing a lot, gaining weight along the 50th percentile, then suddenly reaching the 90th percentile, that tells me they're growing way too fast and they're getting way too many calories.