"Several studies have identified a link between asthma and female infertility, but the impact of asthma treatments on fertility has been unclear". Participants were asked if they had been diagnosed with asthma and, if so, which medicines they had used.
10% of the women questioned had asthma, and researchers found that those who only used their inhaler to alleviate the symptoms of an asthma attack took longer to get pregnant.
They believe the inflammation may also affect the reproductive system.
Those using short acting reliever drugs known as beta agonists took 20 per cent longer to conceive on average compared to healthy peers.
And they were 40 per cent more likely to be classed as "infertile" - defined as having unsuccessfully tried for a baby for more than a year.
All of the participants were expecting their first babies and were in the early stages of pregnancy.
'What we don't yet know is exactly how asthma or asthma treatments lead to fertility problems.
Earlier studies found asthma can harm fertility by causing inflammation in the uterus.
But the same study found that women who took long-acting acting asthma preventers on a regular basis even when they were not experiencing symptoms were as likely to conceive as other women.
'We'd advise women with asthma who are trying for a baby, to ensure they get their asthma under control and to speak to their doctor if they don't have a preventer inhaler'.
Dr Erika Kennington, head of research of Asthma UK, said: 'This study shines a light on how vital it is that people with asthma take their preventative brown inhaler. It could also affect the health of eggs in the ovaries.
"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".
Dr Grzeskowiak added: "There is plenty of evidence maternal asthma has a negative impact on the health of pregnant mothers and their babies and so our general advice is women should take steps to get their asthma under control before trying to conceive".
However, women using only short-acting reliever treatments had a 15% lower rate of fertility than women without asthma.
The researchers acknowledge that they did not recruit women from the time that they started trying to conceive, meaning that the study excluded women who were unable to become pregnant naturally.
According to Asthma UK, 5.4 million people in the UK are now receiving treatment for asthma, a fifth of whom are children.