Researchers said the systemic inflammation that causes asthma may damage fertility by inflaming the uterus, affecting implantation of the fertilised egg.
A study of twins in Denmark found women with a history of asthma were 25% more likely than non-asthmatics to take over a year to fall pregnant.
A team at Bispebjerg University Hospital in Denmark analysed 15,250 female twins and found 955 reported a history of asthma.
Among these women, 27% had tried for more than one year to conceive, compared with 21.6% of non-asthmatics.
Yet, only 23.8% of women whose asthma was well treated suffered fertility problems, compared with 30.5% of untreated asthmatics.
Overall, asthmatics gave birth to the same number of children as non-asthmatics.
Lead author Dr Elisabeth Juul Gade said: '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.'