Women who receive a COVID-19 vaccine may experience a small change in menstrual cycle length, according to a study funded by the National Institutes of Health (NIH).

To assess whether COVID-19 vaccination influences the menstrual cycle, researchers from the Oregon Health & Science University, Portland analyzed data from the Food and Drug Administration (FDA)-approved birth control app Natural Cycles.  

“We included US residents aged 18-45 years with normal cycle lengths (24-38 days) for 3 consecutive cycles before the first vaccine dose followed by vaccine-dose cycles (cycles 4-6) or, if unvaccinated, 6 cycles over a similar time period,” the researchers explained.

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The study included 3959 individuals: 2403 vaccinated and 1556 unvaccinated. Fifty-five percent of the vaccinated group had received the Pfizer-BioNTech vaccine, while 35% and 7% received the Moderna and Janssen vaccines, respectively.

An increase in cycle length of less than 1 day was observed for both vaccine-dose cycles (0.71 day increase after the first dose; 0.91 day increase after the second dose) when compared with prevaccine cycles. No significant changes in cycle length were observed in unvaccinated individuals.

A subgroup analysis showed that participants who received 2 vaccine doses in the same menstrual cycle (n=358) had an average increase in cycle length of 2 days; however, a decrease in subsequent cycles was noted, indicating that the changes were likely temporary. Findings also showed that the vaccine was not associated with changes in menses length.

“Our findings are reassuring; we find no population-level clinically meaningful change in menstrual cycle length associated with COVID-19 vaccination,” the researchers concluded. “Our findings support and help explain the self-reports of changes in cycle length.”


  1. COVID-19 vaccination associated with a small, temporary increase in menstrual cycle length, suggests NIH-funded study. News release. January 6, 2022. https://www.prnewswire.com/news-releases/covid-19-vaccination-associated-with-a-small-temporary-increase-in-menstrual-cycle-length-suggests-nih-funded-study-812551385.html.
  2. Edelman A, Boniface ER, Behar E, et al. Association between menstrual cycle length and coronavirus disease 2019 (COVID-19) vaccination: A US cohort. Obstetrics & Gynecology. Published online January 5, 2022. doi:10.1097/AOG.0000000000004695.

This article originally appeared on MPR