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.
"Five to ten per cent of all women around the world have asthma and it is one of the most common chronic medical conditions 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," Dr Grzeskowiak says.
"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."
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.
Dr Grzeskowiak says: "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."
"There is plenty of evidence that maternal asthma has a negative impact on the health of pregnant mothers and their babies, and so our general advice is that women should take steps to get their asthma under control before trying to conceive," he says.
"What we don't yet know is exactly how asthma or asthma treatments lead to fertility problems. 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."
The researchers plan further studies involving women with asthma who are undergoing fertility treatments, to see whether improving asthma control could also improve fertility outcomes.
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