What does the COVID-19 vaccine do to the menstrual cycle?


A new study showed that COVID-19 vaccinations may temporarily cause changes to the timing of menstruation.

Concerns about the impact of the COVID-19 vaccine on menstrual health have led to much vaccine hesitancy. While its effects weren’t studied in the initial Pfizer/BioNTech, Moderna, and Johnson & Johnson clinical trials, recent research delivers reassuring results.

A retrospective cohort analysis found that slight, temporary changes in menstrual cycles may occur after COVID-19 vaccination but return to normal after 2 postvaccine cycles.1

Investigators conducted a retrospective cohort analysis of nearly 4000 vaccinated and unvaccinated women, examining 6 cycles of menstrual data using the US Food and Drug Administration-approved fertility app Natural Cycles.

Data were used from women in the United States aged 18 to 45. All women reported normal cycle lengths (24 to 38 days), and investigators included data of the vaccinated group for 3 consecutive cycles before the first vaccine dose, followed by vaccine dose cycles (cycles 4-6). Data for the unvaccinated group included 6 cycles in a similar period. They included women in the United States aged 18 to 45 with normal cycle lengths of 24 to 38 days over the course of 3 consecutive cycles.

Of the 3959 women, 2403 were vaccinated and 1556 were not. Most of the vaccinated women received the Pfizer/BioNTech vaccine (55%), followed by Moderna (35%) and Johnson & Johnson/Janssen (7%).

After adjusting for confounders, investigators reported that all women experienced small variations in cycle length, regardless of vaccination status. As for the vaccinated group, the difference in cycle length was less than 1 day when compared with the unvaccinated group. A small group of women who received both vaccine doses in the same cycle had their period for an average of 2 days longer than the unvaccinated group, but this alteration diminished after 2 postvaccine cycles.

Investigators found no change in menses length between or within the 2 groups.

These results are useful for ob-gyns because, even though a 1-day change is not clinically significant, a cycle change of any kind may cause concern or raise hopes of patients avoiding or planning for pregnancy.

This article was originally published by sister publication Contemporary OB/GYN.


  1. Edelman A, Boniface ER, Benhar E, et al. Association between menstrual cycle length and coronavirus disease 2019 (COVID-19) vaccination: a U.S. cohort. Obstet Gynecol. Published online January 5, 2022. doi:10.1097/AOG.0000000000004695
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