One in four premature babies 'faces the risk of autism'

The much higher rate offers one explanation for the increase in children diagnosed with autism in recent years, as early births have become more common.

One in 100 Britons - around half a million - are estimated to be autistic.

Just 20 years ago, scientists estimated that fewer than one in 1,000 people had the condition.

But over the same period, there has been a rise in the number of babies born prematurely and surviving into adulthood.

This is attributed to a combination of medical advances and a trend for more older women, for whom premature babies are more likely, to give birth.

Almost one in eight babies are born prematurely every year, according to baby charity Bliss. That equates to 76,000 babies in the UK.

The Canadian study involved children who were born between seven and 14 weeks prematurely and weighed 3.3lb (1.5kg) or less at birth.

A behavioural test performed at around 21 months of age found suspected autism in 23 of the 91 children.

The study also found that the risk of autism was greatest among those children who were the smallest at birth.

Experts say babies born early may suffer mentally because their brain is still forming.

Previous studies have linked premature birth and low birth weight with developmental problems, including autism.

But the Canadian study is the first to attempt to quantify the risk of autism.

The danger was greatest among those children who were the smallest at birth, and those born to mothers who suffered a prenatal infection or bleeding, said Catherine Limperopoulos, lead researcher of the team from McGill University in Montreal.

"Early autistic behaviours seem to be an under-recognised feature of very low birth-weight infants," she said.

"Ongoing follow-up is needed to understand if this initial positive screen is transient or persistent."

The toddlers who tested as being at high risk for autism were also more likely to be male, and their mother was more likely to have experienced haemorrhaging during labour and delivery.

Autism can have relatively mild symptoms or can severely disable a child by interfering with speech and behaviour.

Higher rates in recent years have been attributed to various causes, including improved diagnoses.

Other studies have found genetic and environmental links to the condition.

But now there is a growing belief that premature births may also be a factor.

The increasing number of mothers who delay giving birth until their forties has been linked in studies to a serious increase in the number of underweight babies.

Most low birth-weight babies are born prematurely - that is, born before 37 weeks' gestation.

Britain has the second worst rate for low birth weights in western Europe - second only to Greece.

The number of over-45s giving birth has doubled in a decade to 1,117 in 2005.

Women over 40 have an 8.5 per cent risk of giving birth to underweight babies, compared with 6 per cent for those in their thirties.

Other studies have shown that premature birth and low birth weight babies are more prone to a low IQ, poor cognitive functioning and learning disabilities.

They are also more likely to exhibit behavioural problems at school.

On top of this, medical advances mean more premature babies are surviving.

Latest figures show that 39 per cent of babies born at 24 weeks live - well up on previous years.

The Canadian study was published in the journal Pediatrics.