Women with asthma who only use short-acting asthma relievers take longer to become pregnant than other women, according to international research led by the University of Adelaide.

However, the study of more than 5600 women in Australia, New Zealand, the United Kingdom and Ireland also shows that women with asthma who use long-acting asthma preventers conceive as quickly as other women.

Published in the European Respiratory Journal, the study was led by Dr Luke Grzeskowiak from the University of Adelaide's Robinson Research Institute.

Dr Grzeskowiak says the results provide reassurance for asthmatic women that using inhaled corticosteroids to prevent symptoms does not appear to reduce fertility.

The researchers examined data from the international Screening for Pregnancy Endpoints (SCOPE) study, which recruited more than 5600 women expecting their first babies in the early stages of pregnancy.

Ten per cent of women in the study said they had asthma and, overall, these women took longer to get pregnant.

When researchers separated this group according to the types of asthma treatments they were using, they found no difference in fertility between women using long-acting asthma treatments and women without asthma.

Women using short-acting reliever medication (known as beta-agonists) took 20% longer to conceive on average. They were also 30% more likely to have taken more than a year to conceive, which the researchers defined as the threshold for infertility.

This difference remained even after researchers took other factors known to influence fertility, such as age and weight, into account.