Women with asthma who only use short-acting reliever medication may take longer to become pregnant than other women, the results of a new international study involving Irish participants has found.

However, it also found that women who use long-acting asthma preventer medication conceive as quickly as other women.

The study involved over 5,000 women in Ireland, the UK, Australia and New Zealand, all of whom were recruited during the early stages of pregnancy.

According to the lead researcher, Dr Luke Grzeskowiak of the University of Adelaide in Australia, 5-10% of all women worldwide have asthma and it is one of the most common chronic conditions found in women of reproductive age.

"Several studies have identified a link between asthma and female infertility, but the impact of asthma treatments on fertility has been unclear.

"Studying the effect of asthma treatments in women who are pregnant or trying to get pregnant is important as women often express concerns about exposing their unborn babies to potentially harmful effects of medications," he explained.

Around 10% of the study participants had asthma and overall, they took longer to get pregnant. However when they were separated into groups based on the types of asthma medication they used, no difference in time to conception was found between those who used long-acting asthma treatments and those without asthma.

On the other hand, women who only used short-acting beta-agonist reliever medication (salbutamol inhaler) took an average of 20% longer to conceive. They were also 30% more likely to have taken over a year to conceive.

The findings stood even when other factors that are known to influence fertility were taken into account, such as age and weight.

"This study shows that women using short-acting asthma relievers take longer to get pregnant. On the other hand, continued use of long-acting asthma preventers to control asthma seems to protect fertility and reduce the time it takes women with asthma to become pregnant. This could lead to a reduction in the need for fertility treatments," Dr Grzeskowiak noted.

He pointed out that it is unknown how exactly asthma treatments can impact fertility.

"As well as affecting the lungs, asthma could cause inflammation elsewhere in the body, including the uterus. It could also affect the health of eggs in the ovaries.

"Inhaled corticosteroids suppress the immune system, whereas short-acting asthma treatments do not alter immune function. In women who are only using relievers it's possible that, while their asthma symptoms may improve, inflammation may still be present in the lungs and other organs in the body," he said.

Further studies in this area are planned.

Commenting on the findings, Prof Mina Gaga, President of the European Respiratory Society, noted that women who are trying to conceive and women who are already pregnant ‘are naturally concerned about the effects of their medicines, although there are large studies showing that asthma medications are safe, in fact safer than not taking medication'.

"This large study provides reassurance that using preventers, which include inhaled corticosteroids and long-acting bronchodilators, to prevent asthma symptoms helps asthmatic women be as fertile as non-asthmatic women, while intermittent treatment with short acting relievers is associated with reduced fertility," she added.

Details of these findings are published in the European Respiratory Journal.