COVID-19 Vaccination Associated With Temporary Small Increase in Menstrual Cycle Length, NIH-Funded Study Suggests

Press release

Thursday, January 6, 2022

Women receiving a dose of a COVID-19 vaccine in a single menstrual cycle had an increase in cycle length of almost a day, compared to unvaccinated women, according to a study funded by the National Institutes of Health. The increased cycle length – a longer time between bleeds – was not associated with any change in the number of menstrual days (bleeding days). The study appears in Obstetrics and Gynecology.

The authors, led by Alison Edelman, MD, MPH, of Oregon Health & Science University, Portland, noted that menstrual cycles generally vary slightly from month to month and the observed increase was well within the normal range of variability. They added that more research is needed to determine how the COVID-19 vaccination could potentially influence other menstrual characteristics, such as associated symptoms (pain, mood changes, etc.) and bleeding characteristics (including the heaviness of the flow).

“It is reassuring that the study found only a small, temporary menstrual change in women,” said Diana W. Bianchi, MD, director of the NIH Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). “These results provide, for the first time, the opportunity to advise women on what to expect from the COVID-19 vaccination so that they can plan accordingly.”

Dr Bianchi added that little research had already been done on how vaccines against COVID-19 or vaccines against other diseases could potentially influence the menstrual cycle.

NICHD and the NIH’s Office of Women’s Health Research funded the study, which was part of $ 1.67 million awarded to five institutions to explore potential links between COVID-19 vaccination and the changes menstrual.

The study authors analyzed anonymized data from a fertility tracking app, Natural Cycles. Users enter data about their temperature and menstrual cycles and can consent to the use of their anonymized data for research purposes. For vaccinated individuals, data were from three consecutive cycles before vaccination and from three consecutive additional cycles, including the cycle (s) in which vaccination took place. For unvaccinated individuals, data were collected for six consecutive cycles. Of the 3,959 individuals in the study, 2,403 were vaccinated and 1,556 were not vaccinated.

Most vaccinated users have received Pfizer and Moderna vaccines. On average, the first dose of vaccination was associated with an increase in cycle length of 0.71 days and the second dose with an increase of 0.91 days. Therefore, users vaccinated over two cycles had an increase of less than one day in each of the vaccination cycles. There was no change in the number of days of menstrual bleeding for the vaccinated individuals. The researchers found no significant change in cycle length for unvaccinated app users.

A subgroup of app users who received two doses of the vaccine in the same menstrual cycle (358 users) had a larger average increase in two-day cycle length. However, this change appears to decrease over subsequent cycles, indicating that the menstrual changes are likely temporary. The authors added that the International Federation of Gynecology and Obstetrics classifies a variation in cycle length as normal if the change is less than eight days.

About Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): NICHD leads research and training to understand human development, improve reproductive health, improve the lives of children and adolescents, and empower people. For more information, visit

About the National Institutes of Health (NIH):The NIH, the national agency for medical research, comprises 27 institutes and centers and is part of the US Department of Health and Human Services. The NIH is the principal federal agency that conducts and supports basic, clinical, and translational medical research, and studies the causes, treatments, and cures for common and rare diseases. For more information about the NIH and its programs, visit

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The references

Edelman, A. et al. Determining the impact of COVID-19 vaccination on the menstrual cycle: an American cohort. Obstetrics and Gynecology. DOI: 10.1097 / AOG.0000000000004695 (2022)


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