Thousands of women with asthma are struggling to become pregnant, because they are taking the wrong medication, according to new research.
Those using short acting reliever drugs known as beta agonists took 20 per cent longer to conceive on average compared to healthy 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 more than 5,000 women in Screening for Pregnancy Endpoints (SCOPE), 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 could slash 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 caesarean section, an underweight baby or a shorter term.
Previous research has suggested it could be due to the steroid medications patients are often prescribed.
Dr Grzeskowiak, a pharmacist, said 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.
Prof Grzeskowiak said: “Five to 10 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.
“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.”
More than one in 10 participants said they had asthma and, overall, these women took longer to get pregnant.
But when the researchers separated this group according to the types of treatments they were using they found the difference was all down to those using beta-agonists.
The disparity remained even after other factors known to influence fertility were taken into account – such as age and weight.
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.”
He added: “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 is 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 acknowledge they did not recruit women from the time they started trying to conceive – meaning the study excluded those who were unable to become pregnant naturally.
They plan further studies involving women with asthma who are undergoing fertility treatments to see if improving control of their condition could also improve fertility outcomes.
Professor Mina Gaga, president of the European Respiratory Society and medical director of the respiratory department of Athens Chest Hospital, said: “Asthma is a common condition but in the majority of cases it can be well-controlled with the right medicines.
“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.”
Last year a study by Swedish scientists of more than a million pregnancies involving over 700,000 women found those with asthma were 17 per cent more likely to suffer pre-eclampsia.
This is a serious condition which affects up to 6 per cent of pregnant women in the UK causing raised blood pressure. It can lead to a stroke in mothers and slow growth in babies if untreated.
At the moment the majority of asthma patients use a drug derived from beta-2 agonists such as Salbutamol.
But these can become ineffective have side effects – like making the immune system less effective – and thereby making it easier for people to catch new infections.
According to Asthma UK, 5.4 million people in the UK are currently receiving treatment for asthma, a fifth of whom are children. In 2016, 1,410 people died from the disease.