Women with asthma may take longer to become pregnant, a new study suggests.

Previous research has indicated a link between the lung condition and fertility, however findings have been inconclusive, so Danish researchers decided to look into this further.

They monitored 245 women, aged between 23 and 45, all of whom had unexplained fertility problems. All of the women underwent fertility treatment and during this time, they filled in questionnaires and underwent asthma and allergy testing.

The women were monitored during their fertility treatment for at least one year until they had a successful pregnancy, stopped treatment or the observation ended.

Ninety-six of the women were already known to have asthma or were diagnosed with the condition during the study.

The researchers found that women without asthma took an average of 32 months to become pregnant. However those with asthma took an average of 55 months.

Those with asthma also had fewer successful pregnancies (39%) compared to women without asthma (60%). The study found that this trend became stronger as the women got older.

"This finding in a clinical trial setting adds new weight to the epidemiological evidence suggesting a link between asthma and fertility. We have seen here that asthma seems to have a negative influence on fertility as it increases time to pregnancy and even more so with age," commented the study's lead author, Dr Elisabeth Juul Gade, of Bispebjerg University Hospital in Copenhagen.

She noted that the reason for the findings is not clear, but said that it is probably ‘complex'.

"Different types of asthma, psychological wellbeing, asthma medication and hormones may all play a role. Given this new evidence, we believe that clinicians should encourage women with asthma to become pregnant at an earlier age and optimise their treatment for asthma pre-conception. Patient education is also of paramount importance as adherence to treatment may be enhanced if patients are informed of this link," she added.

Details of these findings are published in the European Respiratory Journal.