Conceiving less than a year after giving birth - or more than five years later - may increase the child's risk of autism

Babies conceived less than a year after the birth of their sibling are more likely to be diagnosed with autism, according to new research.

The risk of being diagnosed with autism was one and a half times higher if parents conceived a baby within one year of their last baby, than if they conceived a child later, a study found.

The best period to conceive was between two to five years after the previous child, in which there was no extra risk.

Researchers found children conceived after a gap of five years were 30 per cent more likely to be diagnosed with autism.

For gaps of more than ten years, the risk of autism was more than 40 per cent higher.

The study backs up previous research which found a link between conceiving a child closely after having a baby and an autism diagnosis.

Previous studies have shown that women with two closely spaced pregnancies are at risk of premature births and low birth weight.

The study analysed the records of 7371 children born between 1987 and 2005 in Finland, using data from the Finnish Prenatal Study of Autism.

Around a third of the children had been diagnosed with autism.

Researchers used information from national registries to compare the spacing of pregnancies between the children who had been diagnosed with autism and those who had not.

The analysis took into account factors that might explain the link, such as parents' age, prior number of children, and the parent' history of psychiatric disorders like autism.

Dr Alan Brown, of Columbia University in the U.S., said, '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.' 

Dr Cheslack-Postava, of Columbia University, warned the results did not prove that the spacing of pregnancies directly causes autism.

She said: 'It was intriguing to see that the risk of autism 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 autism.

'This is most likely a proxy of other factors that are more directly related to the chance of the child's developing autism.

'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.'

The study was published in the American Academy of Child and Adolescent Psychiatry journal.