December 3 (HealthDay News) -- Children born by Caesarean section are at increased risk of asthma, according to Swiss researchers, who conducted long-term monitoring of the respiratory health of nearly 3,000 infants.

By the time they were 8 years old, 362 (12.4 percent) of the children had been diagnosed with asthma for which they'd been prescribed inhaled steroids. Only 246 (8.5 percent) of the children had been born by Caesarean section, but they were nearly 80 percent more likely to have been diagnosed with asthma than children delivered vaginally, the study found.

The link between Caesarean birth and asthma was even stronger among the nine percent of children who had two allergic parents, and therefore a strong inherited predisposition to asthma. These children were almost three times more likely to be asthmatic by age eight than children whose parents weren't allergic.

The findings were published online Tuesday in the journal Thorax.

The study authors suggest that the increased risk of asthma among children born by C-section may be due to the timing of the "priming" of the immune system, because Caesarean section delays an infant's exposure to microbes.

Rates of asthma in industrialized countries have increased in parallel with a rise in Caesarean section births, which accounted for five percent of births in the 1970s and more than 30 percent of births in 2000, according to background information in the study. Previous research examining the link between asthma and Caesarean delivery has produced conflicting results, but the Swiss team said the number of children in their study, the long monitoring period, and the definition of asthma to include inhaled steroids lends weight to their findings.

"The increased rate of Caesarean section is partly due to maternal demand without medical reason," wrote Dr. Caroline Roduit and colleagues at the Universitats-Kinderlinik in Zurich. "In this situation the mother should be informed of the risk of asthma for her child, especially when the parents have a history of allergy or asthma."

SOURCE: BMJ Specialist Journals, news release, Dec. 2, 2008