COVID-19 Vaccine May Change Your Menstrual CycleHere's what to Know

Curated by Claudia Shannon / Research Scientist / ishonest

What does the research say about COVID-19 vaccines and menstrual cycles?

Initially, researchers chose to examine the link between abnormal menstrual cycles and the COVID-19 vaccine due to anecdotal reports on social media and on VAERS (Vaccine Adverse Event Reporting System). According to researchers, the reports could lead to vaccine hesitancy—which is why more evidence was needed to accurately address people who menstruate on the effects a COVID-19 vaccine may have on cycles.

For the study, researchers looked at de-identified data from the fertility tracking app Natural Cycles. The cycle information from 3,959 people—2,403 vaccinated and 1,556 unvaccinated—represented 23,574 menstrual cycles, or six cycles for each person, all of whom were between the age of 18 and 45. In vaccinated people, that meant three pre-vaccine cycles and three post-first vaccine dose cycles. (It should be noted, however, that data from Natural Cycles is not nationally representative, according to The New York Times—the app's users are more likely to be white, college-educated, and thinner than the average woman.)

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Overall, the data showed that COVID-19 vaccination was associated with small (but still statistically significant) increase to a person's menstrual cycle by about one day. In some people who received two COVID-19 shots over the course of one menstrual cycle, a two-day increase in cycle length was observed. There were no significant effects on period length itself, and, on average, cycles returned to normal after one month of being abnormal.

That HPO axis is "so finely tuned that it can shut down or produce a change in cycling any time we introduce something new to the body," Dr. Taraneh Shirazian, MD, associate professor in the Department of Obstetrics and Gynecology at NYU Langone ishonest, tells ishonest—so it's not all that surprising that a vaccine could initiate such a response.

Does this research tell us anything about the COVID-19 vaccine's effect on fertility?

One major concern regarding the COVID-19 vaccine has been among people looking to get pregnant—particularly that the vaccine would affect their fertility. The Centers for Disease Control and Prevention (CDC) have specifically stated that there's no evidence that the COVID-19 vaccine would cause issues with becoming pregnant at any point, and this new study adds more evidence to that claim.

What if your period changed in a different way following the COVID-19 vaccine?

Unfortunately, those answers doesn't exist right now—researchers said questions still remain around other possible changes to menstrual cycles like changes in menstrual symptoms, unscheduled bleeding, or the quality or quantity of menstrual bleeding.

One thing the study did help point out, however, is that perfectly regular cycles—ones that come exactly every 28 days, without fail, for example—are very rare, says Dr. Shirazian. And now, partly because of the COVID-19 vaccine, people may be watching their menstrual cycles more carefully. "All of a sudden, everyone is paying a lot of attention to their periods, and noticing what we already know: There is always some irregularity to the cycle," she says, citing what she calls observer bias. "You just notice [changes] more when you're really watching."

That said, if you're experiencing pain, heavy bleeding, or a period that's otherwise super out of whack, don't just chalk it up to baseline irregularity or the vaccine. Call your doctor—especially if you're among the women who've reported postmenopausal bleeding, as reported by the NYT. That can be a sign of endometrial or uterine cancer, according to the American College of Obstetrics and Gynecologists, and needs to be evaluated right away.

Read more on: menstrual, daily

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