The COVID-19 Vaccine May Change Your Menstrual Cycle—Here's What to Know

A new study confirms anecdotal evidence from menstruating people.

Soon after the COVID-19 vaccine became available to the general public, people who menstruate began reporting an unexpected side effect: changes to their cycle. Now, research based on those anecdotal claims confirms at least one post-vaccine change to menstruation.

According to a study funded by the National Institutes of Health (NIH), individuals who received one dose of a COVID-19 vaccine saw about a one-day increase in the length of their menstrual cycle—this showed up as a longer time between periods, with periods showing up about one day late, on average. For people who received two doses of an mRNA COVID-19 vaccine in the same cycle, periods were delayed up to two days, study authors said.

The research, published January 5 in the journal Obstetrics & Gynecology, was the first to look at the potential reproductive side effects of COVID-19 vaccines, unrelated to pregnancy. While study authors say more research is needed to examine other potential period-related vaccine side effects—like changes in symptoms or bleeding patterns—the results are still able to provide guidance to menstruating people and reassurance that the slight changes are "within the range of normal variability."

Here, experts help to parse out this new research on COVID-19 vaccines and changes in menstruation, and what they mean for you and your reproductive health.

how the covid vaccine has affected menstrual cycles
Alex Sandoval

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.)

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.

According to the study authors, the vaccine's effect on something called hypothalamic-pituitary-ovarian axis, which helps regulate menstrual cycle timing may be at the root of these changes. The HPO axis "can be affected by life, environment, and health stressors," according to the study—and while researchers said the menstruation changes could not be explained by generalized pandemic stress, they wrote that "mRNA vaccines create a robust immune response or stressor, which could temporarily affect the hypothalamic-pituitary-ovarian axis if timed correctly."

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 Health, tells Health—so it's not all that surprising that a vaccine could initiate such a response.

But while the vaccine can have a short-term effect on periods, it's important to remember that contracting COVID-19 could have an even longer effect: "In the most extreme cases, where you're admitted to the ICU and intubated, you can go months without menses," says Dr. Shirazian. "Severe COVID-19 illness doesn't just change your periods, but can cause reproductive repercussions far worse than one irregular cycle, like miscarriage, preterm labor, and other bad fertility-specific outcomes."

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.

For reference: A normal menstrual cycle lasts between 24 and 38 days, and study authors verify that as much as an eight-day change in cycle length is still considered normal. That means even a one- or two-day change in cycle related to the COVID-19 vaccine is still not of clinical concern, according to researchers.

"If your period is off by a day, that doesn't mean you can't get pregnant," says Dr. Shirazian. "There's no biological mechanism to suggest the vaccine would cause any problems for future fertility. It doesn't affect your eggs. It doesn't affect your ovaries. It doesn't affect your uterus. It's just a small stressor that the body recovers from like anything else."

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.

As for the information we do know right now—regarding menstrual cycle length and delayed periods—researchers say that the changes are insignificant and temporary. "The new data points to no significant change in period patterns with vaccination," says Dr. Shirazian. "It helps us understand that minor cycle changes are possible and common in response to vaccination, so women shouldn't be surprised if their cycles change slightly."

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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