Diagnosis
The diagnosis of FTT relies on plotting the childs height and weight on a validated growth chart, such as the World Health Organization (WHO) growth charts for children younger than two years old or the U.S. Centers for Disease Control and Prevention (CDC) growth charts for patients between the ages of two and twenty years old. While there is no universally accepted definition for failure to thrive, the following are examples of diagnostic criteria for FTT:

Weight under the 5th percentile among children of the same sex and corrected age;
Weight for length below the 5th percentile among children of the same sex and age;
Length for age below the 5th percentile;
Body mass index for age under the 5th percentile;
Weight for age or weight for length dropping by at least two major percentiles (95th, 90th, 75th, 50th, 25th, 10th, and 5th) on a growth chart;
Weight below 75% of the median weight for age;
Weight below 75% of median weight for length; or
Weight velocity less than the 5th percentile.After diagnosis, the underlying cause of FTT must be evaluated by a medical provider through a multifaceted process. This process begins with evaluating the patients medical history. The medical provider will ask about complications during pregnancy and birth, health during early infancy, previous or current medical conditions of the child, and developmental milestones that have been reached or not reached by the child. The childs feeding and diet history, including overall caloric intake and eating habits, is also assessed to help identify potential causes of FTT.
Failure to thrive (FTT), also known as weight faltering or faltering growth, indicates insufficient weight gain or absence of appropriate physical growth in children. FTT is usually defined in terms of weight, and can be evaluated either by a low weight for the childs age, or by a low rate of increase in the weight.The term "failure to thrive" has been used in different ways, as there is no objective standard or universally accepted definition for when to diagnose FTT. One definition describes FTT as a fall in one or more weight centile spaces on a World Health Organization (WHO) growth chart depending on birth weight or when weight is below the 2nd percentile of weight for age irrespective of birth weight. Another definition of FTT is a weight for age that is consistently below the 5th percentile or weight for age that falls by at least two major percentile lines on a growth chart. While weight loss after birth is normal and most babies return to their birth weight by three weeks of age, clinical assessment for FTT is recommended for babies who lose more than 10% of their birth weight or do not return to their birth weight after three weeks. Failure to thrive is not a specific disease, but a sign of inadequate nutrition.In veterinary medicine, FTT is also referred to as ill-thrift.