Study: COVID-19 vaccines may cause small, temporary increase in menstrual cycle

The COVID-19 vaccines from Moderna and Pfizer-BioNTech lengthened the menstrual cycle for some women who received them, according to a new study. File photo by Debbie Hill/UPI
1 of 5 | The COVID-19 vaccines from Moderna and Pfizer-BioNTech lengthened the menstrual cycle for some women who received them, according to a new study. File photo by Debbie Hill/UPI | License Photo

Jan. 6 (UPI) -- Women given their first dose of the two-shot COVID-19 vaccines during a single menstrual cycle experienced an increase in cycle length of nearly one day compared with unvaccinated women, a study published Thursday by Obstetrics and Gynecology found.

The increase in cycle length, or a longer time between bleeding, was not associated with any change in the number of days of menses, or bleeding, the data showed.


Menstrual cycles often vary a small amount from month to month, and the changes seen following COVID-19 vaccination were well within the normal range of variability, the researchers said.

"It is reassuring that the study found only a small, temporary menstrual change in women," Dr. Diana W. Bianchi, director of Eunice Kennedy Shriver National Institute of Child Health and Human Development, said in a press release.

"These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly," Bianchi, whose agency funded the research, which was conducted at Oregon Health and Science University in Portland.


A variation in menstrual cycle length is normal if the change is fewer than eight days, according to the International Federation of Gynecology and Obstetrics.

Most of the more than 2,400 women in the study who were vaccinated against COVID-19 received either the Moderna or Pfizer-BioNTech shots, both of which require two doses, according to Dr. Alison Edelman and her colleagues, who conducted the study.

Additional research is needed to determine how COVID-19 vaccination may affect other menstrual characteristics, such as pain and mood changes, as well as levels of bleeding, Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University, said in a press release.

Clinical trials and subsequent monitoring of vaccine recipients conducted by the Centers for Disease Control and Prevention have found that side effects, aside from injection site pain and fatigue, are rare with the COVID-19 vaccines.

For this study, Edelman and her colleagues analyzed anonymous menstrual cycle data from a fertility tracking app called Natural Cycles.

Users of the app input data on their temperature and their menstrual cycles and consent to the use of their information for research, Edelman said.

Of the 3,959 study participants, 2,403 were vaccinated and 1,556 were unvaccinated, the researchers said.


For vaccinated individuals, data was collected for three consecutive cycles before vaccination and three consecutive cycles, including the cycle or cycles in which COVID-19 vaccination took place, according to the researchers.

For unvaccinated individuals, data was collected for six consecutive cycles, they said.

On average, the first vaccine dose was associated with a 0.71-day increase in cycle length, while the second dose led to a 0.91-day increase, the data showed.

There were no changes in the number of menstrual bleeding days among vaccinated participants and there was no change in cycle length for the unvaccinated, the researchers said.

A subgroup of 358 study participants who received both vaccine doses in the same menstrual cycle saw a two-day increase in cycle length, according to the researchers.

However, this difference appears to resolve in subsequent cycles, suggesting that the menstrual changes most likely are temporary, they said.

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