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Do COVID Vaccines Affect Your Period & Menstrual Cycle?

Essential Takeaways

  • There have been anecdotal reports of COVID-19 vaccines affecting menstrual cycles
  • While we don’t know for sure whether COVID-19 vaccines are causing these changes, experts believe it’s plausible
  • Changes potentially caused by the vaccine should be temporary, lasting only one menstrual cycle, and should not impact fertility

Many women are reporting weird menstrual cycles after vaccination, such as late periods and lighter or heavier flows than normal. These claims drew the attention of Dr. Kate Clancy, an associate professor of Anthropology at the University of Illinois, when she herself experienced a heavier than normal period. “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,” she tweeted

Intrigued, she and her colleague Katharine Lee, a postdoctoral scholar of public health sciences at Washington University, quickly set up an online survey to gather data on menstrual changes after vaccination.

Clancy’s study is still ongoing, but she recently told Salon that her early data show that “[p]eople who have historically menstruated, but are not menstruating now because they are, for instance, trans on gender-affirming hormones, on long-acting reversible contraception, or are postmenopausal… some number of them have experienced bleeding when they had not for a very long period of time.” 

“People who expected to menstruate,” she added, commonly report either “no change,” or “absent or late” or “heavy and early” periods.

Is Vaccination Really Causing These Changes?

Unfortunately, we do not know whether vaccination is affecting women’s menstrual cycles, and we are unlikely to get firm answers soon. 

The problem is that “the variability of the menstrual cycle, even within one woman, is quite large in general,” as Dr. Jerilynn Prior, a professor of endocrinology at the University of British Columbia recently told CBC news

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Because so many women experience irregular periods, we cannot tell whether the reported changes are due to vaccination or something else.

But could vaccination affect your menstrual cycle? Yes, it could. According to Dr. Jen Gunter, a gynecologist and author of the Vagina Bible, vaccination could plausibly affect a woman’s menstruation in a few ways. 

First, vaccination could indirectly affect your period, by altering your stress levels. Perhaps you’re worried about potential vaccine side effects like fatigue, chills, and headaches. Or perhaps your stress levels drop because you are so relieved to finally get vaccinated and to be able to see your family and friends.

Mental and physical stressors like long work hours and illness can definitely affect your menstrual cycle—typically by delaying ovulation, which in turn delays your period. In fact, one study found about a quarter of menstruating women with COVID-19 reported cycle changes, mostly delayed cycles and lighter periods. (Reassuringly, the study also looked at fertility-related hormones, and no changes were seen.)

Another possibility is a direct effect, in which your body’s immune response to the vaccine, which involves inflammation, also affects the uterine lining (endometrium). Normal menstruation and the shedding of the endometrium involves inflammatory signals and activation of several types of immune cells, so this is a plausible biological mechanism.

Thankfully, even if period changes are—indirectly or directly—triggered by vaccination, the effects should be temporary.

When Should I Seek Medical Attention or Treatment?

Because any mild menstrual changes after vaccination are likely minor and temporary, you do not need to seek medical attention if your period is slightly heavier or lighter than normal, or later or earlier than normal, or if you experience light spotting. 

That said, you should never ignore the types of bleeding that would normally lead you to seek medical attention. If you are pregnant and experiencing abnormal bleeding, or post-menopausal and bleeding, or experiencing extremely heavy bleeding, Dr. Margaret Polaneczky, a gynecologist at Weill Cornell Medicine in New York, advises contacting your doctor right away.

Do Menstrual Cycle Changes Imply That the Vaccines Will Harm My Pregnancy or Fertility?

Reports of menstrual changes, associated with the vaccines or not, have made some women understandably skittish about whether the vaccines might affect their pregnancies or chances of conceiving. Misinformation circulating on social media claiming that vaccines or “vaccine-induced shedding” will harm women’s pregnancies or fertility can heighten these concerns.

Fortunately, “there is no evidence that the vaccine can lead to loss of fertility. While fertility was not specifically studied in the clinical trials of the vaccine, no loss of fertility has been reported among trial participants or among the millions who have received the vaccines since their authorization, and no signs of infertility appeared in animal studies,” according to The American College of Obstetricians and Gynecologists (ACOG)

Although we don’t have much direct data from the early vaccine trials, what we do have is reassuring. In the phase 3 trial of the Pfizer-BioNTech COVID vaccine, 23 women conceived during the course of the trial, 11 in the placebo group and 12 in the vaccine group. Two of these women reported adverse events, but both were in the placebo group. 

A recent observational study of women who got vaccinated while pregnant similarly found no signs of elevated rates of pregnancy-related complications or birth defects. 

By contrast, pregnant women do face an elevated risk of developing severe COVID, being hospitalized and needing mechanical ventilation. They also have an elevated risk of preterm delivery.

Should I Still Get Vaccinated if I Am Undergoing IVF or Another Fertility-Related Procedure?

“Patients undergoing fertility treatment and pregnant women should receive the [mRNA] vaccines, according to the American Society of Reproductive Medicine. “Since the [mRNA] vaccine is not a live virus, there is no reason to delay pregnancy attempts because of vaccination administration or to defer treatment until the second dose has been administered.” 

They do, however, make one caveat to this recommendation: Women scheduled for “elective surgery or outpatient procedures, including oocyte retrieval, embryo transfer, and intrauterine insemination, [should] avoid COVID-19 vaccination at least three days prior and three days after their procedure.” This is not because vaccination would interfere with your procedure’s likelihood of success, but because side effects from vaccination such as fever could complicate the monitoring for pre- and post-procedure infections.

What Claims About Menstrual Changes Really Tell Us: Inclusion in Clinical Trials is Not Enough

The menstrual changes reported thus far are not cause for concern, but they are definitely cause for frustration. The unfortunate truth is that we don’t know—and won’t know for a while—whether COVID vaccination causes temporary changes in menstrual cycles, because no one asked about women’s cycles in the vaccine trials. 

Until the 1990s, clinical trials used to exclude women as a matter of course. Thankfully, women’s inclusion in clinical trials for treatments to be used in both sexes is now mandatory. This is undoubtedly a better state of affairs, but inclusion alone is still not sufficient. As Clancy told Salon, “It’s not enough to include [women] if you don’t consider their particular community needs, or their particular differences that make their inclusion necessary.”

As Dr. Guntner wrote in her substack newsletter: “The lack of data regarding menstrual cycles and vaccines is infuriating. I am not concerned about downstream effects regarding infertility or recurrent miscarriages as this has been well-studied for a variety of vaccines. But knowing about menstrual irregularity is as important as knowing about fever.”


View sources

Agostinis, C., Mangogna, A., et al. Uterine Immunity and Microbiota: A Shifting Paradigm. Frontiers in immunology. 2019

Kezhen Li, Ge Chen, et al. Analysis of sex hormones and menstruation in COVID-19 women of child-bearing age. Reproductive BioMedicine Online. 2021

Medical Experts Continue to Assert that COVID Vaccines Do Not Impact Fertility. The American College of Obstetricians and Gynecologists (ACOG), the American Society for Reproductive Medicine (ASRM), and the Society for Maternal-Fetal Medicine (SMFM). 2021

Emergency Use Authorization (EUA) for an Unapproved Product Review Memorandum. 2020.

Shimabukuro, T. T., Kim, S. Y., et al. Preliminary Findings of mRNA Covid-19 Vaccine Safety in Pregnant Persons. The New England Journal of Medicine. 2021

Pregnant and Recently Pregnant People: At increased risk for severe illness from COVID-19. Centers for Disease Control and Prevention. 2021.

Amy Kiefer, PhD

Amy Kiefer is a researcher by training and earned her Ph.D. from the University of Michigan. She currently lives in the Bay Area with her husband and three children where she writes about fertility, pregnancy, and breastfeeding. Check out her blog,

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