COVID Vaccines Create No 'Meaningful Change' in Menstrual Cycles but Do Cause a Brief Shift

Pfizer-BioNTech COVID-19 vaccine
Pfizer-BioNTech COVID-19 vaccine

FRANK AUGSTEIN/POOL/AFP via Getty A person receiving a COVID-19 vaccine

The COVID-19 vaccine can cause slight shifts in menstrual cycles, though nothing long-lasting or worrisome that should stop people from getting vaccinated, a large study has confirmed.

After anecdotal reports popped up on social media from people reporting changes to their menstrual cycles, the National Institutes of Health funded research into possible links between the two, with the first study published Thursday in the journal Obstetrics and Gynecology.

Researchers pulled data from the popular app Natural Cycles, which helps people to track their menstrual cycles and found that the vaccines were associated with a change of less than one day in the length of the menstrual cycle or a shift in when they began — "no population-level clinically meaningful change," they wrote.

"I think it's reassuring and also validating," Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University and lead author of the study, told NPR.

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The researchers looked at data from more than 3,900 Americans — 2,400 of whom received a COVID-19 vaccine and around 1,500 who were unvaccinated — and found that the vaccinated people had, on average, a slightly longer cycle after their first and second doses of Pfizer and Moderna's vaccines. The change was only temporary and resolved within a couple months, they found.

The change is "really trivial," Diana Bianchi, the director of the National Institutes of Health's Eunice Kennedy Shriver National Institute of Child Health and Human Development which helped fund the study, told The Lily, as there is often "normal variation in the menstrual cycles" up to eight days, "and on average this was less than one day."

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Plus, the researchers noted, if people skip vaccination and contract a severe illness like COVID-19 it could be "catastrophic" to menstrual cycles.

The researchers are still unsure, though, exactly why the vaccines led to any shifts. Edelman pointed out that the immune system and the reproductive system are connected, and as the vaccine instructs the immune system to fight COVID-19 it could lead to the change in cycles. They determined that the changes "cannot be explained by generalized pandemic stress."

But, Edelman emphasized, the change was extremely minor and temporary, and should not stop people from getting vaccinated. She also said that the vaccines are entirely safe for people who are expecting or trying to get pregnant.

"We haven't seen anything that's concerning regarding fertility or pregnancy in terms of vaccination," she told NPR, adding that "the risk of COVID-19 disease in pregnant women is incredibly serious."