1. genetic factors

-- about 2 or 3 per cent of brothers and sisters also develop autism; meanwhile about 10-15% of siblings do not develop autism but have other difficulties (language, mental, reading, etc.) These are relatively high rates, suggesting a possible genetic link.

Examples: fragile X syndrome, phenylketonuria, tuberous sclerosis, neurofibromatosis, other chromosomal abnormalities

2. pregnancy/ birth complications

-- certain complications during pregnancy may possibly cause brain damage to the infant (at present, it is unclear whether these cause autism, play a part along with other factors, or are simply indicative symptoms of abnormalities already present)

Examples: bleeding between the fourth and eight month of pregnancy, a 'rhesus incompatibility' between the mother's and child's blood groups, meconium (first stools of the infant) present in the amniotic fluid during labour, mother being above 35 years old at time of child's birth (higher risk), birth order (first or fourth or later-born children may carry a slightly higher risk), medication during pregnancy

3. viral infections

-- some viral infections during pregnancy have been reported in association with autism (like pregnancy complications, it is unclear if these are actually causal of autism)

Examples: congenital rubella, congenital cytomegalovirus, herpes encephalitis

4. metabolic disorders

Examples: abnormalities in purine synthesis, abonormalities in carbohydrate metabolism

5. congenital anomaly syndromes

-- sometimes in children with autism, there are identifiable physical abnormalities at birth, known as congenital anomaly syndromes (eg. an unusually large head, an abnormal formation of ears, etc.)

Examples: Cornelia de Lange syndrome, Noonan syndrome, Coffin Siris syndrome, William's syndrome, Biedl-Bardet syndrome, Moebius' syndrome, Leber's amaurosis

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