Several prenatal and perinatal complications have been reported as possible risk factors for autism. These risk factors include maternal gestational diabetes, maternal and paternal age over 30, bleeding after first trimester, use of prescription medication (e.g. valproate) during pregnancy, and meconium in the amniotic fluid. While research is not conclusive on the relation of these factors to autism, each of these factors has been identified more frequently in autistic children compared to their non-autistic siblings and other normally developing youth.

Low vitamin D levels in early development has been hypothesized as a risk factor for autism.

While infection with rubella during pregnancy causes fewer than 1% of cases of autism, vaccination against rubella can prevent many of those cases.

The results of family and twin studies suggest that genetic factors play a role in the etiology of autism and other pervasive developmental disorders. Studies have consistently found that the prevalence of autism in siblings of autistic children is approximately 15 to 30 times greater than the rate in the general population. In addition, research suggests that there is a much higher concordance rate among monozygotic twins compared to dizygotic twins. It appears that there is no single gene that can account for autism. Instead, there seem to be multiple genes involved, each of which is a risk factor for components of the autism spectrum disorders.

There is no known cure. Children recover occasionally, so that they lose their diagnosis of ASD; this occurs sometimes after intensive treatment and sometimes not. It is not known how often recovery happens; reported rates in unselected samples of children with ASD have ranged from 3% to 25%. Most children with autism acquire language by age five or younger, though a few have developed communication skills in later years. Most children with autism lack social support, meaningful relationships, future employment opportunities or self-determination. Although core difficulties tend to persist, symptoms often become less severe with age.

Few high-quality studies address long-term prognosis. Some adults show modest improvement in communication skills, but a few decline; no study has focused on autism after midlife. Acquiring language before age six, having an IQ above 50, and having a marketable skill all predict better outcomes; independent living is unlikely with severe autism. Most people with autism face significant obstacles in transitioning to adulthood.

There is some evidence that children with AS may see a lessening of symptoms; up to 20% of children may no longer meet the diagnostic criteria as adults, although social and communication difficulties may persist. As of 2006, no studies addressing the long-term outcome of individuals with Asperger syndrome are available and there are no systematic long-term follow-up studies of children with AS. Individuals with AS appear to have normal life expectancy, but have an increased prevalence of comorbid psychiatric conditions, such as major depressive disorder and anxiety disorder that may significantly affect prognosis. Although social impairment may be lifelong, the outcome is generally more positive than with individuals with lower functioning autism spectrum disorders; for example, ASD symptoms are more likely to diminish with time in children with AS or HFA. Most students with AS/HFA have average mathematical ability and test slightly worse in mathematics than in general intelligence, but some are gifted in mathematics. AS has potentially been linked to some accomplishments, such as Vernon L. Smith winning the Nobel Memorial Prize in Economic Sciences; however, Smith is self-diagnosed.

Although many attend regular education classes, some children with AS may utilize special education services because of their social and behavioral difficulties. Adolescents with AS may exhibit ongoing difficulty with self care or organization, and disturbances in social and romantic relationships. Despite high cognitive potential, most young adults with AS remain at home, yet some do marry and work independently. The "different-ness" adolescents experience can be traumatic. Anxiety may stem from preoccupation over possible violations of routines and rituals, from being placed in a situation without a clear schedule or expectations, or from concern with failing in social encounters; the resulting stress may manifest as inattention, withdrawal, reliance on obsessions, hyperactivity, or aggressive or oppositional behavior. Depression is often the result of chronic frustration from repeated failure to engage others socially, and mood disorders requiring treatment may develop. Clinical experience suggests the rate of suicide may be higher among those with AS, but this has not been confirmed by systematic empirical studies.

Education of families is critical in developing strategies for understanding strengths and weaknesses; helping the family to cope improves outcomes in children. Prognosis may be improved by diagnosis at a younger age that allows for early interventions, while interventions in adulthood are valuable but less beneficial. There are legal implications for individuals with AS as they run the risk of exploitation by others and may be unable to comprehend the societal implications of their actions.

Frequency estimates vary enormously. In 2015 it was estimated that 37.2 million people globally are affected. A 2003 review of epidemiological studies of children found autism rates ranging from 0.03 to 4.84 per 1,000, with the ratio of autism to Asperger syndrome ranging from 1.5:1 to 16:1; combining the geometric mean ratio of 5:1 with a conservative prevalence estimate for autism of 1.3 per 1,000 suggests indirectly that the prevalence of AS might be around 0.26 per 1,000. Part of the variance in estimates arises from differences in diagnostic criteria. For example, a relatively small 2007 study of 5,484 eight-year-old children in Finland found 2.9 children per 1,000 met the ICD-10 criteria for an AS diagnosis, 2.7 per 1,000 for Gillberg and Gillberg criteria, 2.5 for DSM-IV, 1.6 for Szatmari "et al.", and 4.3 per 1,000 for the union of the four criteria. Boys seem to be more likely to have AS than girls; estimates of the sex ratio range from 1.6:1 to 4:1, using the Gillberg and Gillberg criteria. Females with autism spectrum disorders may be underdiagnosed.

Anxiety disorder and major depressive disorder are the most common conditions seen at the same time; comorbidity of these in persons with AS is estimated at 65%. Reports have associated AS with medical conditions such as aminoaciduria and ligamentous laxity, but these have been case reports or small studies and no factors have been associated with AS across studies. One study of males with AS found an increased rate of epilepsy and a high rate (51%) of nonverbal learning disorder. AS is associated with tics, Tourette syndrome, and bipolar disorder, and the repetitive behaviors of AS have many similarities with the symptoms of obsessive–compulsive disorder and obsessive–compulsive personality disorder. However many of these studies are based on clinical samples or lack standardized measures; nonetheless, comorbid conditions are relatively common.

A pervasive developmental disorder not otherwise specified (PDD-NOS) is one of the four autism spectrum disorders (ASD) and also one of the five disorders classified as a pervasive developmental disorder (PDD). According to the DSM-IV, PDD-NOS is a diagnosis that is used for "severe and pervasive impairment in the development of reciprocal social interaction or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and activities are present, but the criteria are not met for a specific PDD" or for several other disorders. PDD-NOS is often called atypical autism, because the criteria for autistic disorder are not met, for instance because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these. Even though PDD-NOS is considered milder than typical autism, this is not always true. While some characteristics may be milder, others may be more severe.

The diagnostic category pervasive developmental disorders (PDD), as opposed to specific developmental disorders (SDD), refers to a group of five disorders characterized by delays in the development of multiple basic functions including socialization and communication. The pervasive developmental disorders are: All autism spectrum disorders and Rett syndrome.

The first four of these disorders are commonly called the autism spectrum disorders; the last disorder is much rarer, and is sometimes placed in the autism spectrum and sometimes not.

The onset of pervasive developmental disorders occurs during infancy, but the condition is usually not identified until the child is around three years old. Parents may begin to question the health of their child when developmental milestones are not met, including age appropriate motor movement and speech production.

There is a division among doctors on the use of the term PDD. Many use the term PDD as a short way of saying PDD-NOS (Pervasive developmental disorder not otherwise specified). Others use the general category label of PDD because they are hesitant to diagnose very young children with a specific type of PDD, such as autism. Both approaches contribute to confusion about the term, because the term PDD actually refers to a category of disorders and is not a diagnostic label.

Children with PDD vary widely in abilities, intelligence, and behaviors. Some children do not speak at all, others speak in limited phrases or conversations, and some have relatively normal language development. Repetitive play skills and limited social skills are generally evident as well. Unusual responses to sensory information – loud noises, lights – they also are common.

It is common for individuals with PDD-NOS to have more intact social skills and a lower level of intellectual deficit than individuals with other PDDs. Characteristics of many individuals with PDD-NOS are:

- Communication difficulties (e.g., using and understanding language)

- Difficulty with social behavior

- Difficulty with changes in routines or environments

- Uneven skill development (strengths in some areas and delays in others)

- Unusual play with toys and other objects

- Repetitive body movements or behavior patterns

- Preoccupation with fantasy, such as imaginary friends in childhood

There are many physical health factors associated with developmental disabilities. For some specific syndromes and diagnoses, these are inherent, such as poor heart function in people with Down syndrome. People with severe communication difficulties find it difficult to articulate their health needs, and without adequate support and education might not recognize ill health. Epilepsy, sensory problems (such as poor vision and hearing), obesity and poor dental health are over-represented in this population. Life expectancy among people with developmental disabilities as a group is estimated at 20 years below average, although this is improving with advancements in adaptive and medical technologies, and as people are leading healthier, more fulfilling lives, and some conditions (such as Freeman-Sheldon syndrome) do not impact life expectancy.

The causes of developmental disabilities are varied and remain unknown in a large proportion of cases. Even in cases of known etiology the line between "cause" and "effect" is not always clear, leading to difficulty in categorizing causes.

Genetic factors have long been implicated in the causation of developmental disabilities. There is also a large environmental component to these conditions, and the relative contributions of nature versus nurture have been debated for decades.

Current theories on causation focus on genetic factors, and over 1,000 known genetic conditions include developmental disabilities as a symptom.

Developmental disabilities affect between 1 and 2% of the population in most western countries, although many government sources acknowledge that statistics are flawed in this area. The worldwide proportion of people with developmental disabilities is believed to be approximately 1.4%. It is twice as common in males as in females, and some researchers have found that the prevalence of mild developmental disabilities is likely to be higher in areas of poverty and deprivation, and among people of certain ethnicities.

Specific developmental disorders are disorders in which development is delayed in one specific area or areas, and in which basically all other areas of development are not affected. Specific developmental disorders are as opposed to pervasive developmental disorders that are characterized by delays in the development of multiple basic functions including socialization and communication.

The tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10) has four categories of specific developmental disorder: specific developmental disorders of speech and language, specific developmental disorders of scholastic skills, specific developmental disorder of motor function, and mixed specific developmental disorder.

Education, and a "watch and wait" strategy, are the only treatment needed for many, and the majority of individuals with tics do not seek treatment; treatment of tic disorders is similar to treatment of Tourette syndrome.

Pathological demand avoidance (PDA) is a proposed subtype of autism characterized by an avoidance of demand-framed requests by an individual. It was proposed in 1980 by the UK child psychologist Elizabeth Newson. The Elizabeth Newson Centre in Nottingham, England carries out assessments for the NHS, local authorities and private patients around autism spectrum disorder, which include, but are not exclusively PDA.

PDA behaviours are consistent with autism, but have differences from other autism subtypes diagnoses. It is not recognised by either the DSM-5 or the .

Tic disorders are more common among males than females.

A large, community-based study suggested that over 19% of school-age children have tic disorders; the children with tic disorders in that study were usually undiagnosed.

As many as 1 in 100 people may experience some form of tic disorder, usually before the onset of puberty. Tourette syndrome is the more severe expression of a spectrum of tic disorders, which are thought to be due to the same genetic vulnerability. Nevertheless, most cases of Tourette syndrome are not severe. Although a significant amount of investigative work indicates genetic linkage of the various tic disorders, further study is needed to confirm the relationship.

This is an ill-defined disorder of uncertain nosological validity. The category is included here because of the evidence that children with moderate to severe intellectual disability (IQ below 35) who exhibit major problems in hyperactivity and inattention frequently show stereotyped behaviours; such children tend not to benefit from stimulant drugs (unlike those with an IQ in the normal range) and may exhibit a severe dysphoric reaction (sometimes with psychomotor retardation) when given stimulants; in adolescence the overactivity tends to be replaced by underactivity (a pattern that is not usual in hyperkinetic children with normal intelligence). It is also common for the syndrome to be associated with a variety of developmental delays, either specific or global. The extent to which the behavioural pattern is a function of low IQ or of organic brain damage is not known, neither is it clear whether the disorders in children with mild intellectual disability who show the hyperkinetic syndrome would be better classified here or under F90.- (Hyperkinetic disorders); at present they are included in F90-.

Diagnostic guidelines

Diagnosis depends on the combination of developmentally inappropriate severe overactivity, motor stereotypies, and moderate to severe intellectual disability; all three must be present for the diagnosis. If the diagnostic criteria for F84.0 (childhood autism), F84.1 (atypical autism) or F84.2 (Rett's syndrome) are met, that condition should be diagnosed instead.

Multiple complex developmental disorder is likely to be caused by a number of different various genetic factors. Each individual with MCDD is unique from one another and displays different symptoms. Various neuropsychological disorders can also be found in family members of people with MCDD.

Overactive disorder associated with mental retardation and stereotyped movements is a pervasive developmental disorder (PDD) listed in Chapter V(F) of the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10); its diagnostic code is F84.4.

Multiple complex developmental disorder (MCDD) is a research category, proposed to involve several neurological and psychological symptoms where at least some symptoms are first noticed during early childhood and persist throughout life. It was originally suggested to be a subtype of autistic spectrum disorders (PDD) with co-morbid schizophrenia or another psychotic disorder; however, there is some controversy that not everyone with MCDD meets criteria for both PDD and psychosis. The term "multiplex developmental disorder" was coined by Donald J. Cohen in 1986.

Pathological Demand Avoidance is not recognised by the DSM-5 or ICD-10, the two main classification systems for mental disorder.

To be recognized a sufficient amount of consensus and clinical history needs to be present, and as a newly proposed condition PDA had not met the standard of evidence required at the time of recent revisions. In April 2014 the UK Minister of State for Care and Support Norman Lamb stated that the Department of Health, "In the course of the development of the National Institute for Health and Care Excellence (NICE) clinical guideline on the treatment of autism in children and young people (CG128), the developers looked at differential diagnoses for autism. In this, they did consider PDA, identifying it as a particular subgroup of autism that could also be described as oppositional defiant disorder (ODD). The guidance recommends that consideration should be given to differential diagnoses for autism (including ODD) and whether specific assessments are needed to help interpret the autism history and observations. However, due to the lack of evidence and the fact that the syndrome is not recognised within the DSM or ICD classifications, NICE was unable to develop specific recommendations on the assessment and treatment of PDA."

So the National Institute for Health and Care Excellence (which provides guidelines on best practice for UK clinicians) makes no mention of PDA in its guidelines for diagnosis of autism either in children or adults.

A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered "normal". Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.

There is no known single cause or causes of schizophrenia, however, it is a heritable disorder.

Several environmental factors, including perinatal complications and prenatal maternal infections could cause schizophrenia. These factors in a greater severity or frequency could result in an earlier onset of schizophrenia. Maybe a genetic predisposition is a important factor too, familial illness reported for childhood-onset schizophrenic patients.

Schizophrenia disorders in children are rare. Boys are twice as likely to be diagnosed with childhood schizophrenia. There is often an disproportionately large number of males with childhood schizophrenia, because the age of onset of the disorder is earlier in males than females by about 5 years. People have been and still are reluctant to diagnose schizophrenia early on, primarily due to the stigma attached to it.

While very early-onset schizophrenia is a rare event, with prevalence of about 1:10,000, early-onset schizophrenia is manifests more often with an estimated prevalence of 0.5 %.