Failure to thrive ( FTT ) is a descriptive term given to infants and young children with malnourishment resulting in inadequate growth. To date, there are no universally accepted anthropometric criteria for FTT as different anthropometric indicators and cut points have been used. Commonly used criteria include weight below the 3rd or 5th percentile for age; weight decreasing, crossing two major percentile lines on the growth chart over time; or weight less than 80% of the median weight for the height of the child. Caveats to these definitions exist. According to growth chart standards, 3% of the population naturally falls below the 3rd percentile. These children, who typically have short stature or constitutional delay of growth, usually are proportional (normal weight for height). Additionally, in the first few years of life, large fluctuations in percentile position can occur in normal children. Changes in weight should be assessed in relation to height (length) and head circumference.

Despite these limitations in defining FTT, anthropometric data and growth charts provide important information. Of note, allowances must be made for prematurity; weight corrections are needed until 24 months of age, height corrections until 40 months of age, and head circumference corrections until 18 months of age. Also of note, there are specific growth charts for genetic conditions such as Down syndrome and Turner syndrome that should be used when assessing growth for children with these conditions. Although some growth variants can be difficult to distinguish from FTT, growth velocity and height-for-weight determinations can be useful in distinguishing the cause. In children with FTT, malnutrition initially results in wasting (deficiency in weight gain). Stunting (deficiency in linear growth) generally occurs after months of malnutrition, and head circumference is spared except with chronic, severe malnutrition. FTT that is symmetric (proportional weight, height/length, and head circumference) suggests long-standing malnutrition, chromosomal abnormalities, congenital infection, or teratogenic exposures. Short stature with preserved weight suggests an endocrine etiology.