Seasonally prevalent infectious agents should be expected to cause a seasonal variation in birth month

Professor Henrik Toft Sorensen

They analysed the month of birth of each case and found the greatest number of births took place in August, and the fewest in March.

Previous studies have suggested that exposure to certain infections before birth or in early childhood are important risk factors.

The researchers speculate that infectious agents potentially linked to Crohn's disease may be more common at certain times of year than others.

This variation may lead to seasonal peaks and troughs in the numbers of babies who develop the condition.

Different findings

Regrettably we are still searching avidly for the cause of Crohn's

Dr Simon Travis

The findings contrast with a recent British study, that reported slightly increased risk of Crohn's disease in people born in the first half of the year.

The difference in the findings may reflect the fact that babies are exposed to different infectious agents in the two countries.

Lead researcher Professor Henrik Toft Sorensen, of Aarhus and Aalborg University Hospital, told BBC News Online: "Prior studies have suggested that infections in utero (during the mother�s pregnancy) may be a possible cause for Crohn's disease.

"Therefore seasonally prevalent infectious agents should be expected to cause a seasonal variation in birth month.

"We found such an variation, but our data did not allow us to point out any specific infection."

UK view

Dr Simon Travis is a consultant gastroenterologist at the John Radcliffe Hospital, Oxford, and medical adviser for the National Association for Colitis and Crohn's disease.

He told BBC News Online that the concept of seasonal variation in the risk of Crohn's disease had been studied on many occasions since the mid 1960s.

He said the most reliable study to date was on 420 patients with ulcerative colitis and 142 patients with Crohn's disease in South Eastern Norway.

This identified seasonal variations in the onset of ulcerative colitis, but not of Crohn's disease.

Dr Travis said: "Any association with pre- or perinatal [before or soon after birth] infection must be speculative.

"Indeed, the peak birth rate in mid-summer is rather at odds with what one would expect, because infections are more common in winter months.

"The Danish study is interesting and enters the large body of literature on this topic, but should not be overinterpreted.

"Regrettably we are still searching avidly for the cause of Crohn's: it may turn out to be an infection, but the infection seems more likely to be one of the bacteria ordinarily present in the gut that causes disease in people who are genetically susceptible to the condition."

The research is published in the British Medical Journal.