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 growth 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."
"Failure to thrive" is a scary phrase that pediatricians use when a baby, toddler, or preschooler loses weight. "We're not worried about the short-term effects of a stomach bug, but if your child drops significantly on the weight charts, your doctor should notice," says Lynnette Mazur, M.D., professor of pediatrics at the University of Texas in Houston. Your doc will discuss strategies to improve his diet. Failure to thrive can also indicate an infection, hormonal issues, an allergy, or a chronic disease such as diabetes, so if your child is dropping weight, consult your pediatrician.
Myth My child doesn't sleep well, so he won't grow.
Reality Kids need rest to thrive; their body releases growth hormone during sleep. But even if your child refuses to nap or wakes often at night, he's likely getting enough sleep to grow. If your child is a snorer or a mouth breather or pauses in his breathing when asleep, tell your doctor. These are signs of sleep disordered breathing, a treatable condition affecting about 20 percent of kids.
Myth Your child's weight and height percentiles should match.
Reality Not necessarily, says Parents advisor Ari Brown, M.D., author of Baby 411. "But you don't want your child's weight to be in the 75th percentile and her height in the 10th, since that may indicate she's headed for a weight problem later in life."
Myth Big babies grow into big kids.
Reality "Having a large baby is often just a sign that everything went well during your pregnancy," says Daniel Rauch, M.D., associate professor of pediatrics at Mount Sinai School of Medicine, in New York City. (Your baby may have also been uncharacteristically big if you suffered from gestational diabetes.) But once he's out of the womb, genetics take over, and unless he's got supersize parents, he'll show his true growth patterns between 9 and 18 months.
"Genes are the most important factor, but you can't reach your genetic potential if you're not adequately nourished," explains Dr. Rauch. If she is eating a balanced diet, your child probably doesn't need a multivitamin. However, the American Academy of Pediatrics recommends that all infants, children, and adolescents get a daily dose of 400 IU of vitamin D, which is essential for healthy growth. Since it's hard to get enough vitamin D from food alone, your child will probably need to take a daily supplement soon after she is born, unless she gets at least 34 ounces of D-fortified formula per day.