"What's the right weight for my child?" is one of the most common questions parents have. It seems like a simple one, but it's not always easy to answer.

Growth and Puberty

Not everyone grows and develops on the same schedule. During puberty, the body begins making hormones that spark physical changes like breast development in girls, testicular enlargement in boys, and spurts in height and weight gain in both boys and girls.

These changes continue for several years. The average kid can expect to grow as much as 10 inches (25 centimeters) during puberty before reaching full adult height.

Because some kids start developing as early as age 8 and some not until age 14, it can be normal for two kids who are the same gender and age to have very different weights.

What Is Body Mass Index?

Body mass index, or BMI for short, is a formula that doctors use to estimate how much body fat someone has. The BMI formula uses height and weight measurements to calculate a BMI number. Though the formula is the same for adults and children, figuring out what the BMI number means is a little more complicated for kids.

For kids, BMI is plotted on a growth chart because what is normal changes with age. Different BMI charts are used for boys and girls because growth rate and the amount of body fat differs between boys and girls. Each BMI chart is divided into percentiles that compare measurements with children the same age and gender.

The categories that describe a person's weight are:

Underweight: BMI below the 5th percentile age, gender, and height.

Healthy weight: BMI is equal to or greater than the 5th percentile and less than the 85th percentile for age, gender, and height.

Overweight: BMI at or above the 85th percentile but less than the 95th percentile for age, gender, and height.

Obese: BMI at or above the 95th percentile for age, gender, and height.

Before you calculate your child's BMI, you'll need an accurate height and weight measurement. Bathroom scales and tape measures aren't always precise. So the best way to get accurate measurements is by having kids weighed and measured at a doctor's office or at school.

What Does BMI Tell Us?

You can calculate BMI on your own, but consider asking your doctor to help you figure out what it means. Doctors do more than just use BMI to assess a child's current weight. They also take into account stage of puberty and use BMI results from past years to track whether a child is overweight. Spotting trends early on can be helpful so you can make changes before weight gain becomes a problem.

Overweight and obese kids and teens are developing weight-related health problems, such as type 2 diabetes, high cholesterol, and high blood pressure (hypertension). Overweight teens are also more likely to be overweight as adults. And adults who are overweight have a greater chance of serious health conditions, such as heart disease.

Although BMI can be a good indicator of body fat, it doesn't always tell the full story. Someone with a large frame or a lot of muscle (like a bodybuilder or athlete) can have a high BMI but not too much fat. Likewise, a small person with a small frame may have a normal BMI but could still have too much body fat. These are other good reasons to talk about your child's BMI with your doctor.

When Kids Are Overweight or Underweight

If you think your child has gained too much weight or is too thin, talk to your child's doctor. The doctor has measured your child's height and weight over time and knows whether growth is going as it should.

If concerned about your child's height, weight, or BMI, the doctor may ask questions about your child's health, physical activity, and eating habits, and your family's medical history . The doctor can put all this information together to tell if there's a weight or growth problem.

If your child's weight isn't in the healthy range, the doctor will give you specific diet and exercise recommendations. It's important to follow a plan that's designed for your child by the doctor or a dietitian. For kids and teens, strict diets won't provide the calories and important nutrients their bodies need to grow.

What if your child is too skinny? Most kids who weigh less than others their age are just fine. They may go through puberty on a different schedule than some of their peers, and their bodies may grow and change at a different rate. Most underweight teens catch up in weight as they finish puberty during their later teen years, and there's rarely a need to try to gain weight.

If your child is underweight or losing weight; is tired or ill a lot; has lasting symptoms like a cough, fever, diarrhea, or other problems, talk with your doctor. Kids and teens who are underweight because of eating disorders, like anorexia or bulimia, need medical attention.

The Role of Genes

Heredity plays a role in a person's body shape and weight. But genes are not destiny — kids can reach and keep a healthy weight by eating right and being active.

Genes aren't the only things that family members may share. It's also true that unhealthy eating habits can be passed down. The eating and exercise habits of people in the same household can add to someone's risk of becoming overweight. If parents eat a lot of high-calorie foods or snacks or don't get much exercise, their kids tend to do the same.

The good news is you can change these habits for the better. Even small changes, like cutting back on sugary drinks and going for a walk after dinner, can add up to make a real difference.

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