The risk of delay is greater in women over the age of 30, and whose condition is untreated.
Scientists believe the inflammation characteristic of asthma is likely to have an effect on fertility.
Researchers studied data on 15,000 female twins from Denmark with an average age of 27 who filled in health and lifestyle questionnaires. All the women were asked if they had tried to get pregnant for more than a year without success.
Twins were used in the study to take account of genetic and lifestyle factors that might affect pregnancy.
A total of 995 participants had a history of asthma. Of this group, 27% experienced delayed pregnancy compared with 21.6% of non-asthma sufferers, a significant difference.
Undergoing asthma treatment reduced the chances of waiting for pregnancy to 23.8% compared with 30.5% for women not receiving treatment.
Women above the age of 30 had a stronger tendency towards taking longer to conceive. However, the overall results showed that women with asthma gave birth to the same average number of children as those without the condition.
This was because they were more likely to try for a family at a younger age, when women are more fertile, said the researchers writing in the European Respiratory Journal.
Lead scientist Dr Elisabeth Juul Gade, from Bispebjerg University Hospital in Denmark, 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.”
Deborah Waddell, from the charity Asthma UK, said: “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, and can also call our Asthma UK Helpline on 0800 121 62 44 to speak to a specialist asthma nurse.”