Simply stepping on a scale won't tell you everything you need to know about your health. For example, two 12-year-olds of different heights can both be healthy, even though they weigh two very different amounts. Still, having too much fat on your body can be risky, so your doctor might decide to get more accurate information about your health by measuring your level of body fat.
Because of hormones, your body starts going through big changes around the time you turn 12. Girls start growing breasts, boys start gaining muscle mass and both girls and boys start to get taller as they enter puberty, notes KidsHealth from Nemours. This means you're going to gain some weight, but it's normal and healthy as long as you have the right proportion of muscle, bone and body fat. You already might have started to see some body changes, though you might not begin developing until you're 14. Because of this, no single body fat percentage is ideal for all 12-year-olds. Instead, your doctor will examine you to make sure your body fat percentage is within a healthy range.
Body Mass Index
A formula called body mass index, or BMI, is a helpful tool doctors use to estimate how much body fat you have. Your doctor will take your weight in pounds, multiply it by 703, then divide that number by your height in inches two times, according to the Centers for Disease Control and Prevention. After that, he'll plot where the number falls on a chart for other 12-year-olds of your gender to determine whether your BMI is within a healthy range. You are considered underweight if your BMI is below the 5th percentile, a healthy weight if your BMI is between the 5th and 84th percentiles, overweight if the number lies between the 85th and 94th percentiles and obese if your number is at the 95th percentile or higher.
Your BMI will likely be different from the time you are just turning 12 to the time you are about to be 13. Because of physical changes, the average BMI for both genders gets higher each month. For example, a 12-year-old girl in the 5th percentile might have a BMI number between 14.83 and 15.23, according to the Center for Disease Control and Prevention's Body Mass Index for Age Table. The average BMI for 12-year-old girls -- the 50th percentile -- ranges from 18.01 to 18.63, and the 95th percentile can be 25.27 to 26.13. These numbers are a little different for boys. For example, a 12-year-old boy in the 5th percentile has a BMI of 14.98 to 15.41, while a boy in the 95th percentile has a BMI of 24.23 to 25.1.
Other Types of Measurement
Because BMI is only an estimate of your body fat percentage, your doctor might decide to use a more accurate method if your BMI is very low or very high. One such method is a skinfold estimation, which involves pinching your skin with a tool called calipers to measure how thick it is. If the measurement confirms that your body fat percentage is on the low or high end, your doctor will help you determine what will help you achieve a healthier range. For instance, she might encourage diet and exercise changes, and in some cases she could prescribe medicine.