Experiments have shown a possible link between methods of childbirth and mental health in later life, scientists say.

Doctors have long been puzzled by a statistical link between schizophrenia and birth complications.

Now Canadian researchers, who observed the progress of baby rats born by Caesarean section, think they may have the answer - hormones.

Differences

Their findings were presented at a meeting of the Society for Neuroscience in Los Angeles, and were published in New Scientist magazine.

The researchers performed Caesareans on rats

Although the Caesarean rats seemed normal at birth, the researchers discovered differences between them and vaginal-birth rats.

For example, they observed differences in the dopamine system of the Caesarean rats when compared to the others.

Dopamine is a hormone-like substance that facilitates critical brain functions.

An overactive dopamine system has been linked to schizophrenia and Parkinson's disease since the 1970s.

Amphetamines activate the dopamine system. When given them, the Caesarean rats were much more active than the others.

The Caesarean rats also showed slower reaction times when their tails were pinched.

Hormones

Dr Patricia Boksa carried out the study with Dr Bassem El-Khodor at McGill University in Montreal.

She suggested that the reason behind the differences was that the Caesarean births were not exposed to the surges of hormones present during a vaginal birth.

She said more evidence was needed about the effects of Caesarean sections on babies.

However, she said the findings cast doubt on the "Caesarean section of convenience".

Mr Rupert Fawdry is a fellow of the Royal College of Obstetricians and Gynaecologists and a consultant at Milton Keynes General Hospital.

He said: "It's certainly a most interesting finding. It's one we need to take account of. It's one that may give us some clues as to how and why schizophrenia develops."

However, he said the finding did not mean women should avoid Caesarean sections.

Choices

He said that finding the right method of childbirth was a question of choice based on the available facts, and the research added to those facts.

Women face choices about how to give birth

"It adds to the huge burden we are now putting on women to make choices," he said.

"Every action we take is massively complex and there are no easy answers as to whether it's right or wrong."

However, many women who were familiar with the pros and cons of childbirth methods would opt for a Caesarean, he said.

"A very high proportion of women who have been trained as obstetricians, if asked, would say they would rather have a planned Caesarean."

"It's not a question of convenience - it's a question of choice," he said.

Life factors

The National Schizophrenia Fellowship said it was an interesting piece of research but not a cause for alarm.

Paul Corry, a spokesman for the fellowship, said: "There is a well-proven link between difficulties in birth and a propensity towards schizophrenia.

"Schizophrenia is a very complex illness and having a predisposition to it does not mean you are going to develop it.

"There are usually life events that take place in late adolescence and early adulthood that can trigger episodes of schizophrenia or even life-long illness.