Researchers studied 245 women with unexplained fertility problems aged between 23 and 45 years. They underwent asthma and allergy testing and questionnaires during their fertility treatment.
As many as 96 women in the study had either an existing doctor's diagnosis of asthma or were diagnosed with asthma when they entered the study.
The researchers from Bispebjerg University Hospital in Denmark monitored the women during their fertility treatment for a minimum of 12 months, until they had a successful pregnancy, stopped treatment or the observation ended.
Women with asthma also had fewer successful conceptions - 39.6 per cent achieved pregnancy in the asthmatic women compared with 60.4 per cent in the women without asthma.
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The results also found this trend was more apparent 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," said lead author Elisabeth Juul Gade, from Copenhagen Fertility Centre.
"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," said Gade.
"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," said Gade.
"Patient education is also of paramount importance as adherence to treatment may be enhanced if patients are informed of this link," she said.
The study was published in the European Respiratory Journal.