The developmental history of children with autism seldom reveals medical conditions that can be linked to the disorder. However, certain other illnesses place children at risk for developing autism. These are neurofibromatosis, tuberous sclerosis, and fragile X syndrome. Many children with autism (approximately one fifth to one third) develop seizures. Most of these occur in less able individuals and usually develop in later childhood or adolescence. Many studies have shown that the number of perinatal problems experienced by children with autism is exceptionally high including: difficult delivery, infantile seizures, delayed breathing and neonatal convulsions. Some children appear to have normal development earlier and only show the symptoms of autism in the second year of life. There have been two explanations given for this: (1) the child did show problems earlier but they only became obvious when speech failed to develop and the pressure for socialization was greater, or (2) the child was born with a vulnerability to acquiring the syndrome and it was triggered by a virus or other insult.
As mentioned previously in a very few cases of autism a viral infection in a young child preceded the onset of the symptoms of autism, before which there was a period of apparently normal development. There are also some cases where infections occurred in the mother at an early stage of pregnancy. No other links to environmental conditions have been conclusively proven.
In general, unless there is mental retardation, the signs of autism may not be obvious until the second year of life when language does not develop normally and the child does not show any interest in playing with other children. Stereotypic behaviours may also develop at this time. However, even in early infancy some signs of difficulties may be observable. See the following chart for a list of these signs