(HealthDay News) – The development of children with autism spectrum disorder (ASD) is similar to children without the disorder at 6 months of age, suggesting that ASD has a pre-clinical phase of varying duration when detection may be difficult.

Rebecca J. Landa, PhD, from the Kennedy Krieger Institute in Baltimore, and colleagues regularly examined social, language, and motor development trajectories in 235 children with and without a sibling with autism from 6–36 months of age.

The researchers found that, in comparing children identified as having ASD by 14 months of age, children identified with ASD after 14 months of age, and children without ASD, development was similar at 6 months of age. However, afterwards, the two ASD groups developed differently. At 14–24 months, the early ASD group had greater impairment compared with the late ASD group, but at 36 months, development was similar in both groups. In some children with ASD, developmental plateau and regression occurred, irrespective of the timing of ASD diagnosis.

“The subtlety of early ASD signs and a gradual shift from typical developmental trajectory, particularly children with later manifestation of ASD, may not be detected by ASD screeners or by health professionals in a brief office visit,” Landa and colleagues write. “Thus, parents’ expression of concern about their toddler’s development or behavior requires careful consideration, with referral for stage II screening or developmental assessment if concerns persist for any length of time.”

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