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Baby Playing with Building Blocks

Developmental Milestones

Autism spectrum disorder (ASD) is caused by developmental differences in the brain.


In some children, the first indications​ of ASD may appear before the age of 12 months. In other children, symptoms may not be apparent until 24 months or later. These children may meet developmental milestones, and then they stop gaining new skills at around 18 to 24 months, or they lose the skills they once had.


As the brain develops, individual nerve cells migrate from different starting locations to new destinations where they form connections. The brain's structure is very flexible at this stage in an individual's life, and nerve cells that had formed connections in one area may join a migration to a new area while severing their old connections. The loss of skills observed in some children with ASD may be due to this migration of nerve cells. See the Learning section of our Symptoms and Traits page for more info.


Diagnosing ASD involves reviewing a child's progress through typical developmental milestones as well as the presence of certain restrictive or repetitive behaviors. However, ASD is a dimensional spectrum and not all individuals with ASD display the same combination or severity of symptoms.

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Missed Milestones

The following are milestones that may be missed, delayed, or lost by children with autism. (List adapted from the CDC.)

  • Responding to name by 9 months of age

  • Showing facial expressions like happy, sad, angry, and surprised by 9 months of age

  • Playing simple interactive games like pat-a-cake by 12 months of age

  • Using multiple gestures (e.g., wave goodbye) by 12 months of age

  • Sharing interests with others (e.g., shows you an object that he or she likes) by 15 months of age

  • Pointing or looking at what others point to by 18 months of age

  • Responding when others are hurt or sad by 24 months of age

  • Pretending while playing (e.g., does not pretend to “feed” a doll) by 30 months of age

  • Understanding other’s feelings or talking about own feelings at 36 months of age or older

  • Playing games with turn taking by 60 months of age

  • Progression of speech and listening skills (see this general outline by Yale School of Medicine)

The following are traits that typically progress gradually but may not appear in children with autism.

  • Showing interest in peers

  • Making eye contact

  • Playing with others

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Restricted or Repetitive Behaviors

Other conditions may also cause the same differences in communication and social interaction as autism, so diagnosing autism also involves checking for any of the following restricted or repetitive behaviors.

  • Lines up or arranges toys or other objects and may get upset when they are moved or changed

  • Repeats words or phrases (i.e., echolalia)

  • Plays with things in the same way every time

  • Is more interested in the parts of toys than the toy itself (e.g., likes to spin wheels or play with open/closed doll eyes but does not drive the vehicles or play with the doll)

  • May get upset by minor changes in plans or routines

  • May have stronger interests in a more specific range of topics than most children of a similar age

  • May have specific routines and rituals and may get upset when they are not followed

  • Uses repetitive movements such as flapping hands, rocking body, or spinning in circles to express or regulate emotions or provide sensory input

  • May have strong reactions to the way things sound, smell, taste, look, or feel

  • May not recognize personal space boundaries of others

  • May have a flat or limited facial expressions

  • May speak with a flat, mechanical, or sing-song voice

  • ​May not fully understand jokes, sarcasm, or teasing and may not recognize unfriendly behavior in others

  • May not distinguish between strangers and family/friends

  • May not attend to (look directly at) the faces of others

  • May not recognize emotions in self or others or may exhibit mismatched emotional responses - e.g., laughing when an upsetting event occurs

  • May appear unsure of how to interact or join peers in play or may retreat away from others

  • May be a very picky eater

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