From the Guidelines

The increasing prevalence of Autism Spectrum Disorder (ASD) is likely caused by a combination of genetic and environmental factors.

Genetic factors: Studies have identified many genetic factors that increase the risk of ASD, including rare mutations with large effect 1.
Environmental factors: Exposure to environmental chemicals, such as pesticides, heavy metals, and endocrine disruptors, has been linked to an increased risk of ASD 1.

Prenatal and early postnatal exposure: The early developmental period, particularly the prenatal period, is a sensitive time when the developing brain is susceptible to disruptions from environmental chemicals 1.
Endocrine disruptors: The role of environmental endocrine disruptors, such as bisphenol A (BPA) and phthalates, in the etiology of ASD has been discussed in earlier reviews 1.

Rare genetic variants: The most robustly identified genetic risks for ASD are rare mutations with large effect, which supports the relevance of studying monogenic disorders 1.
Genetic testing: Genetic testing, including chromosomal microarray analysis, is warranted and clinically indicated for all suspected cases of ASD 1.

Heterogeneity of ASD: The heterogeneity of ASD makes it challenging to identify specific causes and develop effective treatments 1.
Further research: Further research is needed to disentangle the heterogeneity of ASD and to identify specific environmental and genetic factors that contribute to the disorder 1.

From the Research

Changes in diagnostic criteria: Studies have suggested that changes in diagnostic criteria, such as the broadening of the diagnostic concept over time, may account for some of the increase in prevalence 2, 3, 4.
Increasing awareness and recognition: Growing awareness and knowledge among parents and professional workers, as well as the development of specialist services, may also contribute to the increase in reported prevalence 2, 3, 4.
Changes in reporting practices: Changes in reporting practices, such as the inclusion of outpatient contacts, can account for a significant portion of the increase in reported prevalence 3.
Genetic factors: Genetic factors are thought to play a major role in the etiology of ASD, with heritability estimates ranging from 0.75 to 0.97 2, 5.
Environmental factors: While environmental factors are also thought to contribute to the etiology of ASD, there is currently no strong evidence to support a significant increase in environmental factors contributing to the rise in prevalence 2, 5, 6.

Vaccines: There is no evidence to support the claim that vaccines, such as the MMR vaccine, contribute to the increase in prevalence of ASD 2.
Secular changes in the environment: There is weak evidence to support changes in genetic and environmental factors underlying ASD and autistic traits over time, suggesting that secular changes in the environment are unlikely to explain the increase in prevalence 5.