There has been conflicting evidence on the link between asthma and C-sections but the researchers said the number of children involved in their study and a long monitoring period strengthened their results.

The findings also underscore the potential risks of elective C-sections as more women in Western countries choose to avoid a natural birth, the researchers said in the medical journal, Thorax.

"The increased rate of Caesarean section is partly due to maternal demand without medical reason," Caroline Roduit of Kinderspital Zurich medical institution and colleagues wrote.

"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."

Asthma, which affects more than 300 million people worldwide, is the most common pediatric chronic illness. Symptoms include wheezing, shortness of breath, coughing and chest tightness.

Babies born by C-section are not exposed to their mother's bacteria when they pass through the birth canal – something that helps prime the immune system and could explain the increased risk, the researchers said.

The Swiss findings are based on nearly 3,000 children whose respiratory health was monitored until age eight. By this time, about 12 per cent, or 362 children, had been diagnosed with asthma for which a doctor had prescribed inhaled steroids.

About 9 per cent of the children were born by C-section but these babies were nearly 80 per cent more likely to develop asthma compared to those born vaginally, the researchers said.

The association was even stronger for the 9 per cent of the children with two allergic parents who were already more predisposed to the respiratory condition, they wrote.

The findings follow a Norwegian study in July suggesting babies born by C-section have a moderately increased asthma risk. Other studies have found no link between C-sections and a child's long-term health, including asthma.