Although it's been recognized for more than a century, failure to thrive lacks a precise definition, in part because it describes a condition rather than a specific disease. Children who fail to thrive don't receive or are unable to take in, retain, or utilize the calories needed to gain weight and grow as expected.
Most diagnoses of failure to thrive are made in infants and toddlers - in the first few years of life - a crucial period of physical and mental development. After birth, a child's brain grows as much in the first year as it will grow during the rest of the child's life. Poor nutrition during this period can have permanent negative effects on a child's mental development.
Whereas the average term baby doubles his or her birth weight by 4 months and triples it at 1 year, children with failure to thrive often don't meet those milestones. Sometimes, a child who starts out "plump" and who shows signs of growing well can begin to fall off in weight gain. After a while, linear (height) growth may slow as well.

Known Causes

Social factors such as poverty can cause failure to thrive for example inadequate living conditions and poor finance. Other factors are, parents with poor parenting skills not giving sufficient or the correct nutrients to a growing baby. Some parents have children on diets, some may be afraid of the child becoming fat or have children with allergies and food intolerances making a balanced diet difficult.
Conditions involving the gastrointestinal system like reflux, chronic diarrhoea, cystic fibrosis, chronic liver disease, and celiac disease can also cause failure to thrive.
Metabolic disorders, which can also limit a child's capacity to make the most of calories consumed.
Infections and chronic illness can also cause a child to not thrive as they may have a poor appetite or be unable to eat.
An intolerance of milk protein. This condition can initially lead to difficulty with absorbing nutrients until it's recognized.

Other Possible Causes

Some professionals have linked adverse reactions to vaccines with failure to thrive. Dr Andrew Wakefield has linked an adverse reaction to the MMR causing some Autistic children to have a chronic bowel disorder, which causes these children to have chronic diarrhoea.

13th July 2008
Reasons to say no to Vaccines
Dr Sherri Tenpenny
http://newswithviews.com/Tenpenny
/sherri19.htm

4th Nov 2007
Children of Pain
http://www.vanguardngr.com
/index.php?option=com
_content&task=view&
id=1457&Itemid=0

Dr Wakefield
http://www.vaccinetruth.org/dr_wakefield.htm

Who Is This Man Wakefield?
http://www.alternative-doctor.com/vaccination/wakefield.htm

Meningitis
http://www.vaccinfo.
karoo.net/page6.html

Adverse reaction to
Meningitis vaccine
http://www.gpnotebook.
co.uk/xda.cfm?ID=80740425