Failure of an infant or young child to gain weight appropriately. The best way to know if your child is gaining weight appropriately is to track his growth (length, weight and head circumference) on a growth chart, which shows both the changes over time, and a comparison to other children of the same age and gender. Your child’s growth will be plotted on such a chart at every well visit to his/her pediatrician. The most common reason that children don't gain weight properly is that they do not receive enough calories, though there are many reasons why this may be so. You might consider keeping a diary of your child's caloric intake so that your child's doctor can compare it to your child's needs. There are some illnesses that result in poor utilization of calories. Your child's doctor can screen for these. They include many medical problems, which are fortunately rare. Chronic infections, hormone problems, and many others may present as failure to thrive. Your pediatrician can distinguish between a normally small child, inadequate caloric intake and medical conditions. Finally, there are some children who are genetically or otherwise destined to be light and short, for example, infants who sustain severe growth restriction before they are born.