Parents should avoid having children too close together, after a study found that babies conceived within 12 months of the birth of a sibling were more likely to be autistic.
Youngsters were 150 per cent more likely to be diagnosed with autism if they were conceived less than a year after their mothers had given birth compared with those born later.
The safest period of conception was found to be between two and five years where there was no extra risk.
But after a five year interval the chance of autism rose again by 30 per cent and a ten year gap increased the risk by 40 per cent.
“This study provides further evidence that environmental factors occurring during or near the prenatal period play a role in autism, a serious and disabling condition that afflicts millions of individuals and that is increasing in prevalence,†said senior author of the study, Dr Alan Brown of Columbia University.
The team claim it is unclear whether the rise in risk is directly caused by the gap, or if other factors which lead to mothers having children more quickly are driving the problem.
But Dr Keely Cheslack-Postava added: “It was intriguing to see that the risk of Autistic Spectrum Disorder diagnosis was higher in both closely and distantly spaced pregnancies.â€
Researchers from Columbia University, US, studied records from more than 7,000 babies born between 1987 and 2005 in Finland. Around one third had been diagnosed with autism.
The study controlled for the age of parents, the prior number of children and previous psychiatric disorders.
They found that babies concieved close to the birth of their brother or sister were at far greater risk of devloping autism.
Around 700,000 people in Britain are believed to have autism, around one in 100 of the population. Around a third of people with a learning disability may also have autism.
Autism can be apparent from an early age with pre-school children struggling with speech devlopment and communication, rejecting affection and friendships and devloping repetitive movements.
However some researchers believe that autism and Asperger’s syndrome may have been over-diagnosed and the problem may not be as widespread as feared.
Professor Uta Frith of University College London believes that many people labelled as autistic may simply have a number of anti-social traits.
She said: “With more lenient criteria and heightened awareness of autism, the diagnostic process will inevitably produce false positives.
“Thus, there are individuals with problems in social relationships and other features that are reminiscent of autism, who have either claimed or been given the label Asperger syndrome, but actually belong to a different category.
My deliberately provocative suggestion is to reconsider the mildest of the mild cases of Asperger syndromes.
“Perhaps these individuals should be classified as having ‘autistic-like personalities’ rather than an autism spectrum disorder.â€
The study was published in the Journal of the American Academy of child and Adolescent Psychiatry.
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