Anecdotal evidence about changes in menstrual cycles for those who got the COVID-19 vaccine has been piling up on social media since the shot became widely available in 2021. People with periods have been reporting longer periods, missed periods, heavier periods, and even, for some post-menopausal women, the return of a period, per the New York Times. But until now, there’s been a lack of large-scale clinical research examining what happens to your period after the COVID-19 vaccine.
A new study published Thursday reports that, confirming at least one post-vaccine period change. Published in the journal Obstetrics & Gynecology, the study found that after getting the COVID-19 vaccine, people with periods had a slightly longer menstrual cycle, with periods coming an average of one day later than normal. (The length of the actual period remained the same.)
Importantly, the change isn’t harmful, according to health experts, and it isn’t permanent. The study found that the menstrual cycle returned to normal one or two periods after getting the vaccine. This falls in line with existing research on the safety of the vaccine for fertility: The COVID-19 vaccine has been proven to have no negative side effects on fertility or pregnancy. The American College of Obstetricians and Gynecologists recommends all people who are pregnant, or plan to become pregnant get the vaccine.
The data on changes in menstrual cycles came from nearly 4,000 women using the Natural Cycles fertility app, about 60% of whom had received the COVID-19 vaccine. That data has one very important drawback, however. As the Times points out, the set of women using the Natural Cycles app isn’t nationally representative—these users tend to be white, educated, thinner than the average American woman, and are not using hormonal contraception, making it difficult to draw generalizable conclusions.
Alison Edelman, M.D., a professor of obstetrics and gynecology at Oregon Health & Science University and the lead author of the study, also noted that there’s lots of room for individual variation in the data. For example, a large factor influencing the findings was a group of 380 women who experienced a two-day delay in the start of their period, she told the Times. A smaller group of vaccinated women—about 5%, per CNN—experienced cycles eight days longer than normal. (However it’s also worth noting that this number was the same among unvaccinated women in the study.) “Though the cycle length was less than one day different at the population level, for an individual, depending on their perspective and what they’re relying on menses for, that could be a big deal,” she said. “You might be expecting a pregnancy, you might be worrying about a pregnancy, you might be wearing white pants.”
The study also doesn’t address reports from post-menopausal women (who aren’t likely to be using the app). Experiencing spotting or bleeding post-menopause can be a cause for concern, whether you’ve received the vaccine or not, and should be evaluated by a doctor.
The study is a step in the direction of understanding how the COVID-19 vaccine—and vaccines in general—impact periods. “It validates that there is something real here,” Hugh Taylor, M.D., chair of the department of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine, who was not involved in the study, told the Times. Adding, “I want to make sure we dissuade people from those untrue myths out there about fertility effects. A cycle or two where periods are thrown off may be annoying, but it’s not going to be harmful in a medical way.”
This post originally appeared on SELF.