New York, Oct 21 (IANS): Younger siblings of children with Autism Spectrum Disorder (ASD) may show signs of the disorder by the time they are just 18 months, said a study.

Different patterns of behaviour at 18 months may be predictive of ASD later on, the findings showed.

"Our research suggests that approximately half the siblings who are later diagnosed with ASD display signs suggestive of ASD at 18 months, and in those who appeared asymptomatic at 18 months, symptoms appeared between 18 and 36 months," said lead author Katarzyna Chawarska, associate professor at the Yale School of Medicine.

The team closely examined social, communicative, and repetitive behaviours in 719 infants when they were 18 months old.

The team looked for patterns that might predict a later diagnosis of ASD. They then followed up when the participants were aged three.

In about 50 percent of siblings, a combination of poor eye contact and lack of communicative gestures or imaginative play is most strongly associated with later ASD diagnosis.

In a small percentage of those later diagnosed with ASD, eye contact may be relatively normal, but they begin to display early signs of repetitive behaviour and have limited non-verbal communication skills.

"So not only do the behavioural symptoms appear at different ages, but different combinations of early symptoms may predict the diagnostic outcome," Chawarska added.

The study appeared in the Journal of the American Academy of Child & Adolescent Psychiatry.