Women with asthma may take longer to get pregnant, a study suggests.
Danish researchers say difficulty conceiving is more likely in those over 30 and when asthma is more severe. They say this suggests that inflammation associated with asthma has an impact on fertility.
UK experts say the findings underline the importance of getting asthma symptoms under control.
Study of twins
Scientists from Bispebjerg University Hospital analysed questionnaires completed by 15,250 Danish twin sisters aged between 12 and 41 who had been asked about their fertility and whether they had asthma.
Twins were used in the study so that genetic and lifestyle factors that might affect pregnancy could be taken into account. The researchers measured how long it took women with asthma to conceive and compared this with the length of time for women who did not have the disease.
Delays in conception
They found that 27% of the women with asthma experienced delays getting pregnant compared with 21.6% of those without asthma. A problem with getting pregnant was defined as trying for a baby for a year without success.
The researchers also noticed an interesting trend in the age of participants. They found that 32.2% of women with asthma above the age of 30 faced delays getting pregnant compared with 24.9% of those with asthma who were under the age of 30.
Undergoing asthma treatment reduced the chances of a delay in pregnancy from 30.5% to 23.8%.
Lead author, Dr Elisabeth Juul Gade, says in a statement: "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."
Notably, the overall results of the study show that women with asthma ultimately gave birth to the same average number of children as women without asthma. Dr Gade says this "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".
The study is published online in the European Respiratory Journal.
Commenting on the findings in an emailed statement, Deborah Waddell, clinical lead at Asthma UK, says: "This study suggests that uncontrolled asthma might be associated with women taking longer to get pregnant, though reassuringly there is also evidence that getting the asthma under control can reduce this delay.
"We know that good asthma management is vital for the health of both mother and baby, and mums-to-be should continue to use their asthma medication during pregnancy. Women who are looking to conceive and are worried about their asthma should speak to their GP or asthma nurse."