Danish researchers looked at large group twin sisters and found that when one of the twins had asthma, her time to conception (TTC) was longer than the sister who did not have asthma. However, this study published in the European Respiratory Journal, the sisters with asthma ultimately had just as many children as their healthier sisters. So, what does this mean? Does asthma harm your fertility? While this is certainly possible, it is just as likely that women with chronic diseases may just take a little longer to get pregnant.