Failure to thrive is a medical term which denotes poor weight gain and physical growth failure over an extended period of time in infancy. Often abbreviated FTT, the term has been in medical use for over a century. As used by pediatricians, it covers poor physical growth of any cause and does not imply abnormal intellectual, social, or emotional development.

In adult medicine, failure to thrive is a descriptive, non-specific term that encompasses "not doing well": e.g. malaise, weight loss, poor self-care that can be seen in elderly individuals.

Recently the term 'faltering growth' has become a popular replacement for 'failure to thrive', which in the minds of some represents a more euphemistic term.

Causes of failure to thrive
Traditionally, causes of FTT have been divided into endogenous, such as an inborn error of metabolism, and exogenous, such as having a mother with postpartum depression. To think of the terms as dichotomous can be misleading, since both endogenous and exogenous factors may co-exist. A child with a disease or disability may be more vulnerable to poor care by a mother with marginal competence or resources. These infants typically look cachectic, are prone to infections with difficulty recovering, are often developmentally delayed, have unusual postures, and look sad, withdrawn, apathetic OR hypervigilant, irritable, or angry. Underlying physical causes may not be immediately obvious such as occult urinary tract infections (UTIs), undiagnosed Cystic Fibrosis (CF) or asthma. Initial investigation should consider physical causes, calorie intake and pyschosocial assessment.

Wider lay use
Like many older medical terms, FTT has acquired some lay connotations which are wider and more pejorative than its strict medical meaning.