Children who are conceived less than one year or more than five years after the birth of their prior sibling appear to be more likely to be diagnosed with autism, according to a new study.

Autism, also known as autism spectrum disorder (ASD), is a biological disorder of the brain that impairs communication and social skills. Signs can include an absence of eye contact, apparent aloofness, avoidance of physical contact even with family, difficulty dealing with interruptions to routine and a lack of interest in other children and what they are doing.

US researchers assessed the records of over 7,300 children who were born in Finland between 1987 and 2005. Around one-third of the children had autism.

The study found that children who were conceived less than one year after their prior sibling's birth were one-and-a-half times more likely to be diagnosed with autism compared to children who were born between two and almost five years after.

Meanwhile, children who were conceived more than five years after their prior sibling were almost 30% more likely to be diagnosed with the condition. For children with a 10-year age gap, the risk of autism was at least 40% higher.

The results stood even when other factors were taken into account, such as parents' age and parental history of psychiatric disorders.

"It was intriguing to see that the risk of ASD diagnosis was higher in both closely and distantly spaced pregnancies. It is important to realise that we can't say from this study that spacing of pregnancies per se is a cause of ASD - this is most likely a proxy of other factors that are more directly related to the chance of the child's developing ASD.

"In other words, the importance of this finding lies in the clues that it can provide in terms of understanding how the prenatal environment is related to outcomes after birth," commented lead researcher, Dr Keely Cheslack-Postava, of Columbia University in New York.

The researchers added that the study provides more evidence that ‘environmental factors occurring during or near the prenatal period play a role in autism'.

Details of these findings are published in the Journal of the American Academy of Child and Adolescent Psychiatry.