COVID-19 vaccines may increase length of menstrual cycle, new study finds

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·5 min read

When people were first able to get the COVID-19 vaccines, some noticed a change in their menstrual cycle. They weren't imagining it: A new study examines how COVID-19 vaccines impact menstruation and found that there is a slight change — less than a day — in the length of the cycle.

“(This research) is both reassuring and validating and provides us a counseling tool. It’s reassuring because we see that at a population level (change) is small,” Dr. Alison Edelman, an author on the paper that appears in the journal Obstetrics & Gynecology, and professor of obstetrics and gynecology at Oregon Health and Science University School of Medicine, told TODAY. “That may mean something different to each individual who menstruates.”

This provides more evidence that COVID-19 vaccines do not impact fertility and their effects on menstruation are short-lived. Many people who are hesitant to receive COVID-19 vaccines worry that it influences their fertility or periods.

“We know that menstruation can be impacted by a variety of factors, including stress, lifestyle changes or a range of underlying conditions," Dr. Christopher M. Zahn, vice president for practice activities at the American College of Obstetricians and Gynecologists said in a statement shared with TODAY. "The new paper published in Obstetrics & Gynecology provides important new evidence underscoring that any impact of the COVID vaccines on menstruation is both minimal and temporary,”

“We continue to stress that the COVID vaccines have absolutely no impact on fertility. People should continue to feel confident in the decision to be vaccinated and, when eligible, to receive a booster, and we encourage everyone aged five and above to get vaccinated.”

Understanding vaccination and menstrual cycle

For the study, Edelman and her colleagues looked at data from 3,959 people with “normal cycle lengths” who used a tracking app called Natural Cycles. Of the participants, 2,403 received either a Pfizer-BioNTech, Moderna or Johnson & Johnson COVID-19 vaccine. The remaining participants, 1,556, had not received a COVID-19 vaccine. The researchers examined three cycles before vaccination and the three during and after.

A healthy cycle is generally considered anywhere from 24 to 38 days and can vary up to eight days. After the first shot, a woman’s cycle was .71 days longer and after the second it was .91 days longer.

“This is really reassuring. In the long run we’re not seeing anything that should prevent somebody from getting vaccinated. It just gives them more information about what to expect,” Edelman said. “Now we can tell folks you might have a slight change in your menstrual cycle length.”

Experts agree that the findings should comfort those worried about infertility.

“One of the major concerns among the unvaccinated were that they were afraid the vaccine was going to affect their fertility,” Christine Metz, professor at the Feinstein Institutes for Medical Research at Northwell Health, who wasn’t involved in the research, told TODAY. “What this paper shows is that menstrual cycle length, which is a sign of a healthy reproductive state, is not changed by the vaccine. This is small, 7/10 of a day, change.”

Dr. Diana Bianchi, director of the National Institutes of Health Eunice Kennedy Shriver National Institute of Child Health and Human Development, said that this study’s design strengthens the findings.

“This included a control group, which is important,” she told TODAY. “Women who were vaccinated are matched to women who were not vaccinated, on the basis of their age, their race, etc. And that is how they could tell that unvaccinated women did not have this effect.”

Though, Metz said the study has some limitations. The app costs money to use, which reduces the diversity of the populations who use it.

“There were more white people overrepresented in this group. More college-educated people overrepresented in the group,” she said, also adding that some people who use the app are tracking their fertility for family planning or using it as a non-hormonal birth control.

What’s more, this study didn’t examine something commonly known to impact menstruation: stress.

“The authors did not assess stress,” Metz said. “There could be stress related to the vaccine as well as obviously the stress of the pandemic. Maybe the people who were vaccinated had more stress related to the pandemic than those who didn’t.”

Though Bianchi thinks that if stress impacted the menstrual cycle the control group might have experienced changes, too.

The future of period research

There’s still little research on how any vaccine impacts periods.

After people first observed changes in their bleeding after receiving their vaccines in the spring, researchers Katharine Lee and Kathryn Clancy created a survey to capture the changes people experienced.

“We need to do more work noticing when there are different effects for different people, really, so that we can do a better job of (preparing for) these side effects,” Clancy, of the Clancy Lab at the University of Illinois, told TODAY in April 2021. “If people know, for instance, this is going to make you bleed more they’re going to have more pads with them.”

They’re still examining the data they've collected.

“We have mostly been focusing on our analyses thus far on people describing heavier periods,” Lee, the postdoctoral research scholar in the public health sciences division at the Washington University School of Medicine told TODAY. “A fair number of (people) seem to be experiencing a heavier period after vaccinations — for just a couple of periods.”

Lee said she hopes that research about vaccines and menstruation help people understand what could occur.

“Just being prepared that your period might be different than what you expect and not necessarily knowing which direction it might go,” she said. “We have tons of people who reported they didn’t have any difference in bleeding.”

Bianchi hopes that future vaccine and drug development research considers menstruation, which has been overlooked in the past.

“This really is an opportunity for us to recognize something new about women’s biology and we need to follow up on this with other studies of vaccines as well as therapeutics,” she said. “It would be very simple just to add a few questions (to vaccine surveys) about the menstrual cycle. So my hope is that those questions will be added to future investigations.”