COVID shots can impact period timing, studies suggest. Women were already wondering

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Throughout the COVID-19 pandemic, women turned to TikTok and Twitter to document something strange: The coronavirus vaccines seemed to be impacting their menstrual cycles.

People with periods shared stories of heavier flows, cramping and changes to their cycle timing, including periods starting late or lasting longer.

Menstruation can be a tough area to research — since various factors can affect someone's period, from stress to medical issues — but there is now a growing body of research suggesting some of those early pandemic hunches were on to something.

A new study featuring data from nearly 20,000 people around the world, including thousands of Canadians, showed individuals who received any of nine different vaccines experienced close to a one-day average increase in cycle length.

Released in late September by the U.S. National Institutes of Health (NIH), and published in the British Medical Journal, the study suggested people who received two COVID vaccine doses in a single menstrual cycle experienced a nearly four-day increase — though the changes were not associated with any difference in the number of days of actual bleeding.

The length of a menstrual cycle can vary from person to person, from around 21 to 35 or more days, but the average is to have periods every 28 days. Bleeding typically lasts anywhere from two days to a week.

Roughly three-quarters of the participants were vaccinated, while around a quarter were not, and all of them were users of the same app to track their menstrual cycles. The researchers analyzed data on at least three consecutive cycles before vaccination, plus at least one cycle after. 

"We think that perhaps an acute inflammatory response, like you get after a vaccination, could influence those cells within the uterus, and that could be changing menstruation," said NIH researcher Candace Tingen, one of the scientists involved in the study, during an interview with CBC News.

The cycle changes also appeared to be temporary, she said.

"And so the menstrual cycle goes back to its normal pattern, its normal rhythm, within a few cycles."

Any change in cycle length of less than eight days is considered within the normal range.

Although small menstrual changes may not mean much to medical professionals, the study authors wrote, perceived changes in a bodily function linked to fertility could be alarming for those experiencing it, and may contribute to vaccine hesitancy.

That's exactly how Toronto resident Kaitlan Rowe felt after both she and her 15-year-old daughter experienced changes in their menstrual cycles after having their second round of Pfizer COVID-19 shots last summer.