Study links Covid-19 vaccination to small temporary change in menstrual cycle


People who received the Covid-19 vaccine reported slightly longer menstrual cycles, but the change was temporary, according to a new study published Tuesday in the journal BMJ Medicine.

“Compared to the unvaccinated group, vaccinated individuals had an adjusted increase in menstrual cycle length of less than a day with the first and second doses of vaccine,” wrote the study authors, Universities of Oregon, Massachusetts, London and Edinburgh. “Individuals who received two doses of a covid-19 vaccine in a single cycle had an adjusted increase in cycle length of 3.70 days compared to the unvaccinated.”

The vaccinated cohort had a change in cycle length of about 0.71 days after their first vaccine dose, the authors said.

There was also a “significant increase” in the rate of responders who had an increase of more than eight days in their cycle length, the authors said, finding that 13.5% of those vaccinated and 5% of participants unvaccinated reported it.

“We found no difference in menstrual period duration in any group of vaccinated individuals, compared to the unvaccinated cohort,” the study says.

Changes in cycle length did not stay in the cycle after vaccination, the authors said – except in the group that received two doses in one cycle.

The changes appeared to be similar regardless of which vaccine a person received.

The authors looked at nearly 20,000 people, representing over 250,000 cycles, who recorded their data using the Natural Cycles app between October 1, 2020 and November 7, 2021. For the vaccinated cohort, they reviewed three pre-vaccination cycles and at least the first Covid -19 vaccine dose cycle. For the unvaccinated, they included four to six cycles of a similar period.

The participants came from several countries, but especially from Europe, the United States and Canada. Nearly two-thirds of the 15,000 vaccinated participants had received the Pfizer/BioNTech vaccine, although participants also received the Johnson & Johnson, Moderna and Oxford-AstraZeneca vaccines, among others.

“These results provide additional information to advise women on what to expect after vaccination,” said Dr. Diana Bianchi, director of the National Institute of Child Health and Human Development (NICHD) at the National Institute of Health Eunice Kennedy Shriver, in a press release. “Changes after vaccination appear to be small, within the normal range of variation, and temporary.”

According to the authors, the research has some limitations, including that people using hormonal contraceptives were not participants, that researchers were limited in the number of post-vaccination cycles, and that they were unable to consider the effects of potential infections in participants.

The findings align with an earlier study published by the same group of researchers that focused on women in the United States.

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