Women with asthma could be taking longer to become pregnant, due to the effects of their medication, according to new research.
An Australian study showed that women using short acting reliever drugs - known as beta agonists - took 20 percent longer to conceive on average compared to peers.
They were also 30 per cent more likely to have been trying for a baby for more than a year to conceive - seen as the threshold for suffering infertility.
But there was no difference in fertility between patients on inhaled steroids, known as longer acting preventer drugs and women without asthma.
The study was based on over 5,000 women in SCOPE (Screening for Pregnancy Endpoints) an international trial of first time pregnancies in Australia, New Zealand, the UK and Ireland.
Dr Luke Grzeskowiak, of the Robinson Research Institute at Adelaide University, said it slashed the number of asthmatics requiring fertility treatment.
He said: "This study shows 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."
Asthma is known to increase the risk of complications in pregnancy including the potentially fatal condition pre-eclampsia.
Sufferers are also more likely to have a cesarean section, an underweight baby and a shorter term.
Previous research has suggested it could be due to the steroid medications patients are often prescribed.
Prof Grzeskowiak, a pharmacist, says his results provide reassurance for asthmatic women that inhaled corticosteroids to prevent symptoms does not seem to reduce fertility.
The study published in the European Respiratory Journal examined data from SCOPE in which participants were asked if they had been diagnosed with asthma and what medicines they had used.
They were also asked how long it had taken them to become pregnant. Those using steroids conceived as quickly as other women.
According to Asthma UK, 5.4 million people in the UK are currently receiving treatment for asthma, a fifth of which are children (1 in 11). In 2016, 1,410 people died from the disease.
The study was welcomed by Britain's leading asthma charity.
Dr Erika Kennington, head of research at Asthma UK, said: "This study shines a light on how vital it is people with asthma take their preventative brown inhaler.
"For the three million women in the UK who have asthma this will not only build up protection over time in their airways - preventing them from having a potentially fatal asthma attack - but this study suggests it also means their fertility will be less likely to be affected.
"At Asthma UK we'd encourage everyone with asthma to take all their medicines including their preventer inhaler.
"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."
The study published in the European Respiratory Journal was based on over 5,000 women in SCOPE (Screening for Pregnancy Endpoints) an international trial of first time pregnancies in Australia, New Zealand, the UK and Ireland.