In the first months of the pandemic, scientists were quick to rule out changes in menstrual cycles—heaviness, duration, pain—as possible side effects of the vaccine, despite thousands of women reporting them. But the reports eventually were taken seriously, and institutions including the National Institutes of Health (NIH) funded research to look into potential links.
An NIH-funded study published Jan. 5 in the journal Obstetrics and Gynecology confirmed that covid-19 vaccines have an effect—if minor—on the duration of menstrual cycles, making the period start slightly later than expected. These changes are temporary, and don’t impact fertility in any way, though it is important for women to know that they might experience slight changes in their period following covid-19 immunizations.
Studying the impact of an event on menstrual cycles isn’t so easy because it requires tracking prior to the event, as well as ruling out all other causes, which also is complicated because everything from stress to illness to diet can affect a cycle.
In this case, the researchers used period-tracking data collected by users through an app called Natural Cycles. About 4,000 met the requirements to be included in the study, accounting for nearly 24,000 menstrual cycles.
App users included in the study had received Pfizer (55% of users), Moderna (35%), and Johnson & Johnson covid-19 vaccines (7%); 3% of users didn’t know which vaccine they had received. Women who had received the AstraZeneca vaccine, were menopausal, or suffered from conditions including endometriosis or polycystic ovarian syndrome were excluded from the study.
The researchers found that when women got the vaccine, their next menstrual cycle was longer than usual, on average by just under a day. The changes—which extended the overall duration of the cycle (which can be between 28 and 35 days) but not the length of the period—occurred after each shot received.
“This means that the time between a person’s first day of bleeding until their next first day of bleeding may be slightly longer than what they consider ‘normal,'” explains Alison Edelman, the obstetrics and gynecology professor at Oregon Health & Science University who led the study.
The research didn’t look into other reported changes in periods, but Edelman said researchers plan to investigate other reported changes, such as heavier flows or other symptoms associated with menstruating.