Many babies go through brief periods when their weight gain plateaus or they even lose a little weight. However,
if a baby doesn't gain weight for 3 consecutive months during the first year of life, doctors usually become concerned.
Doctors diagnose failure to thrive by using standard
growth charts to plot weight, length, and head circumference, which are measured at each well-baby exam. Children who fall below a certain weight range for their age or who are failing to gain weight at the expected rate will be evaluated further to determine if there's a problem.
Along with obtaining a thorough medical and feeding history and performing a detailed physical exam, the doctor may order a complete blood count, urine test, and various blood chemical and electrolyte tests to search for underlying medical problems. If a particular disease or disorder is suspected, the doctor might perform additional tests specific to that condition.