Failure to thrive describes children whose weight or rate of weight gain is far below that of other children of the same age and sex. The condition has many causes. For example, children who fail to thrive may not be receiving the calories they need to gain weight and grow as expected. Or, their bodies may not be able to take in, keep or use those calories properly. Most diagnoses of failure to thrive are made in the first few years of life - a crucial period of physical and mental development.