Explanation:
Most authors define failure to thrive as crossing two major weight percentiles on the appropriate growth chart. Remember to use common sense when using this definition. A baby born heavy is heavy due to maternal and placental influences. These children may cross several percentile lines before reaching their appropriate genetic percentiles. If an infant is referred for failure to thrive and they have substantial fat stores, there is little reason for concern!

Other sources may define failure to thrive as a weight below the third percentile or the failure to grow at a rate consistent with the appropriate growth chart.

Although failure to thrive can be associated with crossing percentiles for length and head circumference in severe/prolonged cases, the term failure to thrive refers primarily to weight.

Children should regain their birth weight by 14 days of age and double their birth weight by 6 months of age. Infants younger than 3 months of age should gain 25-30 grams per day. However, these rules of thumb are not the criteria for failure to thrive.

Remember that failure to thrive is not a diagnosis. It is similar to anemia in that it defines a problem, but not the specific underlying cause.