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. Twins were used in this research, as they not only enable direct comparisons to be made between twin sisters, but also comprise a sample representative of the whole population, being born into all social groups and avoiding the need to measure genetic and lifestyle information for each individual.
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.
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% of asthmatics vs. 21.6% 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% of untreated asthma group vs. 23.8% 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% women above the age of 30 vs. 24.9% 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.
Lead author, Dr Elisabeth Juul Gade, said: "Our results shed light on the complex interactions between fertility and asthma." 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."
"Despite the delay, our overall results suggest that women with asthma had the same number of children, which is due to the fact that they tend to conceive at an earlier age compared to those without, getting a head start on their reproductive life."