Women are sharing personal stories about this on social media, so we asked experts if period changes could be a side effect of vaccination.

The COVID-19 vaccine can come with many (totally normal) side effects, like a fever, chills, and muscle pain. So it's only natural to wonder if it can temporarily influence other aspects of your health, like your menstrual cycle.

First, there's no reason not to get vaccinated when you have your period; neither the CDC nor the American College of Obstetricians and Gynecologists (ACOG) lists being on your period as a reason to put off the vaccine.

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But in terms of how the vaccine affects menstruation in general, some women have taken to social media to talk about how they're suddenly experiencing heavier-than-normal periods after getting vaccinated.

In a  lengthy Twitter thread, University of Illinois associate professor Kate Clancy, PhD, had this to say: "A colleague told me she has heard from others that their periods were heavy post-vax. I'm curious whether other menstruators have noticed changes too? I'm a week and a half out from dose 1 of Moderna, got my period maybe a day or so early, and am gushing like I'm in my 20s again."

Other people chimed in with their own stories. "I am exactly 1 week after my Moderna 2nd shot and I started a very heavy cycle for me, and im about 2.5 weeks early," one wrote. "I got the Moderna on the last day of my period. Nine (!!!!) days later another period appeared and has been a nightmare," commented another.

The personal stories continued. "I am 3 weeks out from my first Moderna shot and started my period in the middle of a [birth control] pack," one wrote. "That's never happened to me in 12 years of taking the pill. I never even spot between periods."

Clancy said in a later tweet that her period "wasn't nearly as bad as after the first shot," and that she expects that her next period will "be back to what's typical for me." She also said that she and her colleague are planning to study this phenomenon further.

So are period changes a legit side effect of the vaccine? Here's what you need to know.

Is there any research on COVID-19 vaccines and menstrual cycles?

Not really. The Centers for Disease Control and Prevention (CDC) has a Vaccine Adverse Event Reporting System, which is a surveillance program where anyone can report the side effects they experienced after getting the vaccine. Several dozen women reported having heavier-than-usual periods, painful cramps, and unusual menstrual cycles after being vaccinated. But given that anyone can submit anything to the system, it's all just anecdotal at this point.

OK, but what could be going on here?

Experts say it's really hard to know for sure. For those women who report more pain than usual, "it may be that aches and pains post-vaccine compound normal menstrual pains," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security in Maryland, tells Health.

The heavier flow is a little trickier to explain. A small study of 233 women of childbearing age with clinically diagnosed COVID-19 reported some menstrual changes. Of the 177 patients with records about their periods, 25% had "menstrual volume changes," 20% had a lighter-than-usual period, and 19% had a longer-than-usual cycle, according to the study, which was published in January in the journal Reproductive BioMedicine Online. While it's possible that the COVID-19 vaccine could have a similar effect, it's not known at this time.

There's also the possibility that stress could play a role. Stress can be an annoying and somewhat vague excuse for period-related changes, but it's a real possibility, Gloria A. Bachmann, MD, associate dean for women's health at the Rutgers Robert Wood Johnson Medical School in New Jersey, tells Health. "Menstrual cycles can be altered or influenced by many factors, including stress, poor sleep, exercise, and some medications," she says. "Therefore, it wouldn't be that unusual for some women to notice, after receiving the vaccination, changes in their period, such as it coming on earlier, or having a heavier flow, or noticing more cramping than they usually have."

The idea that a vaccine might affect a woman's cycle is a tough one to explain from a biological standpoint, though, Mark Turrentine, MD, professor of obstetrics and gynecology at Baylor College of Medicine in Texas, tells Health. "There is no biologic mechanism that would account for [the] disruption of the menstrual cycle following receipt of the COVID-19 vaccine," he says. Dr. Turrentine also points out that unusual vaginal bleeding "was not a side effect reported in any of the clinical trials from the vaccine manufacturers," adding, "no large-scale adverse events regarding irregular menstrual bleeding have been noted to date."

Despite the personal stories from women online, women's health expert Jennifer Wider, MD, tells Health that there just isn't enough data to show that the vaccine could impact menstrual cycles. "Could the vaccine interact with the body's hormones or are there other factors at play like stress? That remains to be seen," she says.

What happens if my period changes after I get vaccinated?

First, don't panic. While it could just be one of those things that can occur following a vaccine, it also may be a sign of something else going on that just coincidentally showed up after you were vaccinated.

If the unexplained change persists, call your doctor. "Although the menstrual changes can be due to the COVID-19 vaccination, it's always better to discuss the situation with your health care provider as certain evaluation and tests, such as a gynecologic exam or pelvic ultrasound, may be indicated," Dr. Bachmann says.

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 CDCWHO, and their local public health department as resources.

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