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Two researchers have launched a study to determine whether Covid-19 mRNA vaccines cause abnormal periods such as heavy bleeding.

Dr. Kathryn Clancy and Dr. Katherine Lee say they launched the study when they both experienced heavy menstrual bleeding after taking the vaccines.

When Clancy and Lee asked their Twitter followers to describe their abnormal menstrual cycles after taking the vaccines, hundreds of women replied.

Clancy and Lee are now compiling data from a questionnaire asking women to describe their abnormal periods. As of Monday, more than 25,000 women have completed the questionnaire, Dailymail.com reported.

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In a series of tweets in February, Clancy discussed her heavy menstrual bleeding after taking the first shot of Moderna.

"I'm curious whether other menstruators [sic] have noticed changes too?" she wrote.

"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. I'm on day 3 of my period and am still swapping out extra long overnight pads a few times a day. Typical for me at this time is maybe one or two regular pads."

On April 7, Clancy tweeted out a link to a survey in which women could document their menstrual irregularities.

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Thrombocytopenia, or low blood platelets, was among the adverse effects reported to the Vaccine Adverse Event Reporting System (VAERS) system.

Low blood platelets causes abnormal bleeding, weakness and fatigue.

Experts say Pfizer, Moderna and Johnson & Johnson did not report any menstrual irregularities during their relatively short clinical trials.

They say it's too early to suggest a link between heavy cycles and vaccinations until a larger study is conducted.

The Food & Drug Administration approved the mRNA vaccines on an emergency basis.

The FDA approval allowed pharmaceutical companies to forego longterm studies of their vaccines.