Overt features gradually begin after the age of six months, become established by age two or three years, and tend to continue through adulthood, although often in more muted form. Some of the early signs of ASDs in this course include decreased attention at faces, failure to obviously respond when name is called, failure to show interests by showing or pointing, and delayed imaginative play. Regressive autism development
Regressive autism occurs when a child appears to develop typically but then starts to lose speech and social skills and is subsequently diagnosed with ASD. Other terms used to describe regression in children with autism are autism with regression, autistic regression, setback-type autism, and acquired autistic syndrome.Within the regressive autism developmental course, there are two patterns. The first pattern is when developmental losses occur in the first 15 months to 3 years. The second pattern, childhood disintegrative disorder (a diagnosis now included under ASD), is characterized by regression after normal development in the first 3 to 4, or even up to 9 years of life.After the regression, the child follows the standard pattern of autistic neurological development. The term regressive autism refers to the appearance that neurological development has reversed; it is actually only the affected developmental skills, rather than the neurology as a whole, that regresses. The apparent onset of regressive autism can be surprising and distressing to parents, who often initially suspect severe hearing loss. Attribution of regression to environmental stress factors may result in a delay in diagnosis.There is no standard definition for regression.
(For more, see Brett Abrahams)

Genetics
Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by rare mutations with major effects, or by rare multi-gene interactions of common genetic variants. Complexity arises due to interactions among multiple genes, the environment, and epigenetic factors which do not change DNA sequencing but are heritable and influence gene expression. Many genes have been associated with autism through sequencing the genomes of affected individuals and their parents. However, most of the mutations that increase autism risk have not been identified. Typically, autism cannot be traced to a Mendelian (single-gene) mutation or to a single chromosome abnormality, and none of the genetic syndromes associated with ASD have been shown to selectively cause ASD. Numerous candidate genes have been located, with only small effects attributable to any particular gene. Most loci individually explain less than 1% of cases of autism. As of 2018, it appeared that between 74% and 93% of ASD risk is heritable. After an older child is diagnosed with ASD, 7–20% of subsequent children are likely to be as well. If parents have one autistic child, they have a 2% to 8% chance of having a second child who is also autistic. If the autistic child is an identical twin the other will be affected 36 to 95 percent of the time. If they are fraternal twins the other will only be affected up to 31 percent of the time.