The development of children with Autism Spectrum Disorder (ASD) is much like that of children without ASD at 6 months of age, but differs afterwards. That's the main finding of the largest prospective, longitudinal study to date comparing children with early and later diagnosis of ASD with children without ASD. The study appears in the journal Child Development and has implications for clinical work, public health, and policy. The study was conducted by researchers at the Kennedy Krieger Institute, the Johns Hopkins University School of Medicine, the Johns Hopkins Bloomberg School of Public Health, and the Aging Brain Center at the Institute for Aging Research and Hebrew SeniorLife at Harvard Medical School.

The study sought to learn more about the patterns of development during the first three years of life in children with and without ASD to better understand how ASD can be detected as early as possible. It is the first prospective study to examine early-onset ASD (by 14 months) and later-onset ASD (after 14 months) over the first three years, pinpointing where development looks the same and where it diverges（分歧，偏离）.

ASD comprises a group of disorders of brain development that affects about 1 in 88 American children.

Researchers looked at 235 primarily White children with and without an older sibling with autism, testing them at regular intervals from ages 6 to 36 months. Using standardized and play-based assessments, they tested children's fine motor skills, understanding of spoken language, and spoken language production skills. They also measured how often the children shared their emotions and initiated communication with others.

The study looked at early development across three groups: children without ASD, children with ASD who were identified by 14 months, and children with ASD identified after 14 months. At 6 months, development within the early- and later-identified ASD groups was comparable to each other and to the non-ASD group. At 14 and 18 months, the early-identified ASD group performed below the later-identified ASD group in many aspects of development. By 24 to 36 months, the two groups showed similar levels of development.

"Results show that ASD has a preclinical phase when detecting it may be difficult," explains Rebecca Landa, director of the Kennedy Krieger Institute's Center for Autism and Related Disorders and the study's lead author. "In some children with ASD, early signs of developmental disruption may not be ASD-specific.

"Routinely administering general developmental screeners, such as the Ages and Stages Questionnaire, should begin in infancy, complemented by ASD-specific screeners by 14 months," suggests Landa. "Screening should be repeated through early childhood. If concerning signs of delay associated with ASD are observed in a child who scores normally on standardized tests, further assessment is warranted."