Some women may have more or fewer asthma symptoms, such as coughing and wheezing, depending on their time of the month, a new study suggests.
Researchers said spikes and dips in oestrogen and other hormones likely affect the lungs and other physiological responses involved in breathing. However, it's still unclear whether the findings could improve doctors' treatment of women with asthma.
The menstrual cycle "is a very important cycle... with all the biological changes and physiological things that happen," said Dr Samar Farha from the Cleveland Clinic, who studies asthma and other respiratory diseases but wasn't involved in the new research.
"(Some) asthmatics describe that just before their menses, they get a worsening of their symptoms," she told Reuters Health - but more scientific assessments of what's going on have come to conflicting conclusions.
For the new study, researchers surveyed close to 4,000 women in Northern Europe who had normal periods and weren't taking birth control pills.
Along with other health and lifestyle questions, they asked women to report when their last period started, as well as whether they'd had any breathing-related problems in the past three days - such as wheezing or waking up with a coughing attack.
Just under eight per cent of women in the study had been diagnosed with asthma. Between two and six per cent reported recent wheezing, coughing and/or shortness of breath.
Dr Ferenc Macsali of Haukeland University Hospital in Bergen, Norway, and colleagues found the number of women with of each of those symptoms changed depending on where they were in their menstrual cycle.
For example, wheezing spiked just before and just after mid-cycle (ovulation). The dip in between corresponds to peaks in oestrogen, follicle stimulating hormone and luteinising hormone, the researchers write in the American Journal of Respiratory and Critical Care Medicine.
Complaints of shortness of breath and coughing both declined just after women got their periods, and shortness of breath was also more rare right before menses started.
Macsali's team saw cyclical patterns in breathing symptoms in women with and without asthma.
What explains those patterns is still up for debate. Oestrogen may affect the lungs directly, the researchers said. Insulin resistance and markers of general inflammation are known to vary during the menstrual cycle, which could also play a role in when breathing symptoms get better or worse.
"The observed patterns in our study are most likely a result of... complex hormonal processes," the researchers wrote, "and it does not seem plausible that one sex hormone should explain the variation in respiratory symptoms during the menstrual cycle."
Women with asthma should "be aware of a possibility that their symptoms are influenced by day in cycle," Macsali told Reuters Health in an email.
Not all asthmatics will notice those changes, Farha pointed out.
For women whose symptoms do get a little worse at certain times of the month, it's also unclear whether that ever puts them in serious danger, she added. But Farha said it may still be worth bringing up the issue with their doctor.
"It could lead to more personalised therapy, based on where their symptoms are getting worse, in which phases of their menstrual cycle," Farha said. "You could change therapy and escalate therapy based on (those) phases."