Researchers from Bispebjerg University Hospital in Denmark analysed data from questionnaires completed by a cohort of over 15,000 twins living in Denmark aged up to 41 years.
The questionnaires included questions on the presence of asthma and on fertility. The researchers divided the participants into women with asthma and those without, and then sub-divided the groups into those treated for asthma and those not treated for asthma.
All participants were asked whether they had been trying to get pregnant for longer than a year without success and how many children they had given birth to. Around 955 of the participants reported a history of asthma.
The results found a significantly higher proportion of women who experienced a prolonged time to pregnancy in the group with asthma, compared to those who did not have asthma (27 per cent of asthmatics vs 21.6 per cent of non-asthmatics).
The risk of a delay in conceiving significantly increased in women with untreated asthma compared to those with asthma who were undergoing treatment (30.5 per cent of untreated asthma group vs 23.8 per cent of those receiving treatment). The researchers also noticed an interesting trend in the age of participants.
Women above the age of 30 with asthma had an even stronger tendency towards a long waiting time to pregnancy (32.2 per cent women above the age of 30 vs 24.9 per cent of women under the age of 30). However, the overall results of the study showed that women with asthma ultimately gave birth to the same average number of children as women without asthma, as those with asthma tended to have children earlier in life than those without asthma.
"Our results shed light on the complex interactions between fertility and asthma," said lead author, Dr Elisabeth Juul Gade. "Although we observed women with asthma experiencing longer waiting times to pregnancy, our findings suggest that if women take their medication and control their asthma, they can reduce this delay.
"As the negative effect of asthma on fertility is reduced by treatment, we can assume that the systemic inflammation characterised by asthma may account for the effect on delaying fertility," Gade said.
The study was published in the European Respiratory Journal.