BBC News' James Westhead reports on the findings

A study published in The Lancet medical journal found that couples were less likely to achieve success during fertilisation treatment if the male partner had been in regular contact with pesticides.

There has been growing concern about the effect pesticides may have on the human reproductive system, and particularly on the quality of men's sperm.

Regular contact

The new research found that out of 650 couples undergoing in-vitro fertilisation (IVF) treatment, 20 men were found to have had regular contact with pesticides.

Seven of these - including four fruit farmers and two flower growers - had been exposed to high levels of pesticides.

Compared to other couples undergoing IVF, these men were significantly less successful in achieving fertilisation.

Those who had the highest exposure had the least sucessful fertilisation rates.

"Our results are the first to suggest that paternal pesticide exposure decreases the sperm fertilising ability in vitro (in a test tube)," the Dutch researchers said.

BBC Health Correspondent Richard Hannaford said the survey was too small to allow definite conclusions to be made.

However, it will increase the pressure for more detailed research to be carried out, he said.

Previous findings

Earlier research has also suggested that exposure to pesticides may affect men's sperm.

DDT metabolite pesticides have been shown to block the action of male hormones.

And workers exposed to high levels of the pesticide chlordecone are known to suffer decreased sperm motility and abnormal sperm.

The authors of the new study, however, could not say which pesticides were to blame.

"Because most individuals were exposed to multiple pesticides with various active ingredients, it is impossible to draw conclusions as to which chemical may be responsible for that effect," they said.