Girls and boys are measured on different growth charts because they grow in different patterns and at different rates. And one set of charts is used for babies, from birth to 36 months. Another set of charts is used for kids ages 2 to 20 years old. Also, special growth charts are used for children with certain conditions, like Down syndrome.

Percentiles are measurements that show where a child is compared with others. On the growth charts, the percentiles are shown as lines drawn in curved patterns. When a doctor puts a child’s weight and height on the chart, they see which percentile line those measurements land on. The higher the percentile number, the bigger a child is compared with other kids of the same age and gender, whether it’s for height or weight. The lower the percentile number, the smaller the child is.

For example, if a 6 month-old boy’s weight is in the 10th percentile, that means that 10% of boys that age weigh less than he does and 90% of 6 month-old boys weigh more.

Ideally, a baby will follow along the same growth pattern over time, growing in height and gaining weight at the same rate, with the height and weight in proportion to one another. This means that usually a child stays on a certain percentile line on the growth curve. So if our 6 month-old boy on the 10th percentile line has always been on that line, he is continuing to grow along his pattern, which is a good sign.

What might be a problem:

• When a child’s weight or height percentile changes from a certain pattern it’s been following.
• When kids don’t get taller at the same rate at which they’re gaining weight.