Researchers are investigating whether there is a link between COVID-19 vaccines and abnormal periods after hundreds of inoculated women reported the possible side effect.
Kathryn Clancy, an associate professor at the University of Illinois and Katharine Lee, a postdoctoral research scholar at Washington University School of Medicine, both noted changes to their periods after receiving the jab.
After posting to Twitter asking if other women had experienced similar effects, Clancy became inundated with messages.
"I ended up finding a lot of people with similar experiences," Clancy told the Chicago Tribune. "But also, to be fair, a lot of people who were like, 'I noticed nothing', and some people who said, 'actually I had the opposite, where I've had a later or lighter period'."
Illinois woman Katy Fyksen, 43, hadn't had a period in over a year-and-a-half due to using the IUD contraceptive.
But after receiving her second dose of Moderna's COVID-19 vaccine, she surprisingly got her period.
"I didn't really think it was anything until I saw someone had said that it might've been a symptom or a side effect of the vaccine. It was like, 'oh, that's interesting'," she said.
After receiving the feedback, Clancy and Lee launched a survey to catalogue people's menstrual experiences after receiving the vaccine - and it has already been filled out by more than 25,000 people.
The survey will look to see if any trends require further investigation, Lee told the Chicago Tribune.
"Our survey cannot tell us anything about prevalence or the number of people who are affected," Lee said. "What we can do is look for associations and trends that help us direct whatever the next study would be."
But experts say there are still no signs of danger associated with the vaccines, and people shouldn't take this as a reason not to get vaccinated.
Dr Jane Kelly, South Carolina's assistant state epidemiologist, told WCNC it is "hard to say" whether periods and the vaccine could be related.
"Pfizer, Moderna, Janssen - they all had more than 40,000 people participating, and they did not see any reports of changes to menstrual cycles."
She said other factors could also play a part in the menstrual changes.
"There's so many factors that can influence a woman's menstrual cycle including physical and mental stress," Kelly said.