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Vaccinated Women Had a Longer Cycle Than Those Not Vaccinated

— January 18, 2022

On average, vaccinated women have a slightly longer menstrual cycle than those not vaccinated.

A new study, the first of its kind, shows that women who received one dose of the coronavirus vaccine during a single menstrual cycle had an average increase in their cycle length of approximately one day compared women who are not unvaccinated.  The increase was associated with a longer time between bleeding and was not related to any change in the number of days that they are on their period.  The study was published in the Obstetrics & Gynecology Journal.

Alison Edelman, M.D., M.P.H., of Oregon Health & Science University, Portland, indicated that cycles of the women differed just a few days from each month.   The team also noted that additional research is needed to determine how vaccination status might influence other aspects of the cycle, such as premenstrual pain and changes in mood associated with premenstrual syndrome well as an increase in bleeding, although the team didn’t necessarily note a change in the number of days with menstrual period bleeding.

Vaccinated Women Had a Longer Cycle Than Those Not Vaccinated
Photo by Andrea Piacquadio from Pexels

“It is reassuring that the study found only a small, temporary menstrual change in women,” said Diana W. Bianchi, M.D., director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). “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.”  It is important for women to monitor their cycles for any changes, especially if these changes do not go away within the first couple of menstrual periods and to follow up with their doctors if substantial changes are noted.

The study authors analyzed data from Natural Cycles, an app that has an an algorithm that determines fertility status based on basal body temperature. Women can also use it to track their cycles.  For vaccinated individuals, the reported data was from “three consecutive cycles before vaccination and from three more consecutive cycles,” according to the study..  For unvaccinated women, data was collected for six consecutive cycles as well.  The authors indicated that “2,403 participants were vaccinated and 1,556 were unvaccinated.”

The majority of vaccinated women received the Pfizer or Moderna immunizations.  For most, their first dose caused a “.71-day cycle increase in cycle length and the second dose with a .91-day increase,” the authors reported.

Therefore, users vaccinated over two cycles had an increase of less than one day in each of the vaccination cycles.  There were no changes to the number of menses days for those vaccinated. The researchers noted no significant changes in cycle length for those unvaccinated users as well.  Another group of Natural Cycle users who received two vaccine doses in the same cycle (i.e., roughly 360 women) had a higher average rise in cycle length over two days.  However, this change decreased in future cycles.

The authors added that the International Federation of Gynecology and Obstetrics considers a change in cycle length as normal but this should not extend beyond eight days.  In general, they concluded that having a coronavirus disease vaccine ran parallel with a small change cycle length but no significant change to menses days.


Small, temporary increase in menstrual cycle length linked to COVID-19 vaccine: Study

Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination


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