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Female-to-male trans (FtM) individuals report irregular periods after receiving the Covid-19 mRNA vaccine.

FtM trans whose menstrual periods stopped while on the male hormone testosterone now complain that they are menstruating again.

Biological women in the LGBT and heterosexual communities report irregular menstruation such as heavy bleeding after taking Covid-19 mRNA vaccines.

A few FtM trans took to social media to vent their frustration.

One FtM trans who hasn't had a period in 5 years started menstruating spontaneously within days of taking the Moderna injection.

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A heterosexual woman wrote that her 24-year-old biological sister, who self-identifies as a male, began bleeding heavily after taking the vaccine.

"He was very unprepared, and in shock at Aunt Mary's return," she wrote in a post on Reddit.com. "Aunt Mary" is slang for menstruation.

She added: "I'm worried it may affect my cycle if I get it? I'm very regular; you could set a clock to my cycle."

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

200 million adults and children in America have been vaccinated -- more than half of the U.S. population.

But the pace of vaccinations has begun to slow, because people who don't want the vaccine are not seeking appointments.

The White House and the CDC are concerned the U.S. might not reach herd immunity, as those who are fully vaccinated begin to fall ill from Covid-19.

In many states, unused vaccines are beginning to pile up, leaving thousands of vaccines to spoil.

While New Hampshire has given at least one shot to 59 percent of its citizens, states such as Mississippi, Alabama, Georgia and Florida are at or below 30 percent vaccinated.

"There are states where they feel they have hit the wall," said Michael Fraser, the executive director of the Association of State and Territorial Health Officials. "The folks that wanted [vaccines] have found it. The folks that don't want it are not bothering to find it."

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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.