Update: May 9, 2023
In a recent study supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), researchers found that women have a slightly greater probability of a heavier period after being vaccinated for COVID-19. However, menstrual flow returned to normal in the next menstrual cycle.
Researchers used self-reported menstrual cycle data from more than 9,000 people through the Natural Cycles fertility tracking app. The researchers found that a higher percentage of vaccinated people experienced an increase in bleeding during the menstrual cycle after their first dose (38.4%) than unvaccinated people (34.5%). This means that about 40 out of every 1,000 people could experience heavier bleeding after COVID-19 vaccination.
The researchers found that there was no difference in the number of heavy bleeding days between vaccinated and unvaccinated people, and by the next menstrual cycle, there was no difference in bleeding quantity. These results are consistent with other research that indicates a small and temporary change in menstrual flow after COVID-19 vaccination.
Update: October 5, 2022
In an international study of data from nearly 20,000 people who received any of nine different COVID-19 vaccines, researchers have confirmed a link between COVID-19 vaccination and an average increase in menstrual cycle length of less than one day. This increase was not associated with a change in the number of days of bleeding, and for most study participants, the increase resolved in the menstrual cycle following vaccination.
The study was supported by by NICHD and NIH’s Office of Research on Women’s Health. These results confirm the findings of a previous U.S. study conducted by the same principal researcher and supported by NICHD (see below). The results of the international study were first published in September 2022.
Update: January 20, 2022
In a study supported by NICHD, researchers found that the length of the menstrual cycle — the time between periods — temporarily increased by an average of less than one day in people who received a dose of the COVID-19 vaccine, compared with unvaccinated people. Vaccination was not associated with a change in the number of days of bleeding.
The study’s authors examined the menstrual cycles of more than 2,400 vaccinated people and more than 1,500 unvaccinated people. A small group of people who received both vaccine doses in the same menstrual cycle had an average two-day increase in cycle length compared with unvaccinated people.
For the vaccinated group, changes went away in later menstrual cycles, indicating that the changes in cycle length are temporary. The International Federation of Gynecology and Obstetrics states that changes in menstrual cycle length are considered normal if the change is less than eight days.
“It is reassuring that the study found only a small, temporary menstrual change in women,” said Diana W. Bianchi, M.D., director of 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.” Future research will assess whether the COVID-19 vaccine affects other aspects of the menstrual cycle, such as amount of bleeding and menstrual symptoms, such as pain or mood changes.
Update: October 5, 2021
NICHD recently awarded five institutions one-year supplemental grants totaling $1.67 million to explore potential links between COVID-19 vaccination and menstrual changes. Researchers at Boston University, Harvard Medical School, Johns Hopkins University, Michigan State University, and Oregon Health and Science University will investigate whether such changes may be linked to the COVID-19 vaccine itself or if they are coincidental, the mechanism underlying any vaccine-related changes, and how long any changes last.
Several of these studies will use blood, tissue, and saliva samples collected before and after vaccination to analyze any immune or hormone changes. Other studies will use established resources — such as large cohort studies and menstrual cycle tracking apps — to collect and analyze data from racially, ethnically, and geographically diverse populations. Two studies will focus on specific populations, including adolescents and people with endometriosis.
What you need to know
Increased stress, changes in weight and exercise, and other major lifestyle changes can affect menstrual cycles — and all of those changes are common during the COVID-19 pandemic. Additionally, studies have shown that some women who had COVID-19 experienced changes in the duration and flow of their menstrual cycles.
Some people have reported changes in their menstruation after receiving the COVID-19 vaccine, including changes in duration, flow, and accompanying symptoms such as pain.
What will researchers be doing?
To learn whether there is a connection between vaccination and changes in menstruation, the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) recently released a notice of special interest for researchers to compare the menstruation experiences of vaccinated and unvaccinated people. NICHD will support research focused on menstruation before and after vaccination and how vaccination as well as other factors, such as stress, might influence menstrual changes.
Why is this research important?
As more people are vaccinated for COVID-19, it is possible to gain better understanding of short- and long-term effects of the vaccines. Scientific evidence could also help unvaccinated people understand what, if any, menstruation-related side effects to expect from a COVID-19 vaccine.
Where can I go to learn more?
- NICHD calls on researchers to study the possible effects of the COVID-19 vaccine on menstruation.
- NICHD shares information about menstruation and menstrual cycle irregularities.
Darney, B. G., Boniface, E. R., Van Lamsweerde, A., Han, K., Matteson, K. A., Cameron, S., Male, V., Acuna, J., Benhar, E., Pearson, J. T., & Edelman, A. (2023, April 10). Impact of coronavirus disease 2019 (COVID-19) vaccination on menstrual bleeding quantity: An observational cohort study. BJOG: An International Journey of Obstetrics & Gynaecology. https://doi.org/10.1111/1471-0528.17471
Edelman, A., Boniface, E. R., Benhar, E., Han, L., Matteson, K. A., Favaro, C., Pearson, J. T., & Darney, B. G. (2022). Association between menstrual cycle length and coronavirus disease 2019 (COVID-19) vaccination: a U.S. cohort. Obstetrics and Gynecology, 10.1097/AOG.0000000000004695. Advance online publication. https://doi.org/10.1097/AOG.0000000000004695
Edelman, A., Boniface, E. R., Male, V., Cameron, S. T., Benhar, E., Han, L., Matteson, K. A., Van Lamsweerde, A., Pearson, J. T., & Darney, B. G. Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data. BMJ Medicine 2022;1:e000297. https://doi.org/10.1136/bmjmed-2022-000297
Li, K., Chen, G., Hou, H., Liao, Q., Chen, J., Bai, H., Lee, S., Wang, C., Li, H., Cheng, L., & Ai, J. (2021). Analysis of sex hormones and menstruation in COVID-19 women of child-bearing age. Reproductive Biomedicine Online, 42(1), 260–267. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7522626/
This article was originally published on August 2, 2021.
NIH COVID-19 Resources by Topic
COVID-19 research information and resources by topic from NIH institutes and centers