They're an easy and accurate way to track how your child is growing. At each checkup, your doctor will measure your child's height, weight, and -- if he's younger than 2 -- head size and plot the results on a chart to compare him with other kids the same age and sex. So if your 2-year-old son is in the 75th percentile for height and 50th percentile for weight, that means he is taller than 75 percent of boys his age and weighs more than half of them do.
The Centers for Disease Control and Prevention (CDC) recently recommended that doctors use the 2006 charts made by the World Health Organization (WHO) until your child turns 2 and then switch over to the CDC's charts. The WHO charts are based on predominantly breastfed infants with access to good nutrition, and they best reflect how your child should be growing under ideal conditions. The CDC also has a chart for tracking body mass index (BMI), which is the best predictor as he gets older of whether he's at a healthy weight.
Don't stress about your child's specific percentile: "It doesn't really indicate how well she's growing," says Richard Ball, M.D., a pediatrician at Akron Children's Hospital in Akron, Ohio. "The key is whether she is consistently at the same percentile. If she slows down in length, for example, or shoots up in weight, we're more likely to be concerned."