Receiving at least one COVID-19 vaccine dose can slightly increase the length of a woman's menstrual cycle, according to a large study funded by the National Institutes of Health.
Vaccinated women experienced an increase in their menstrual cycle, the time between periods, on average by less than one day, compared to unvaccinated women, though the change typically resolved one cycle after vaccination, the study published in BMJ Medicine found."These findings provide additional information for counseling women on what to expect after vaccination,” said Diana Bianchi, director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development.
The international study led by Dr. Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University, analyzed data from a fertility tracking app for nearly 20,000 people from Canada, the United Kingdom, the United States, and Europe, among other places. It found that vaccinated women saw an average cycle increase of .71 days after the first dose and a .56-day increase after the second dose.
The changes in cycle length did not differ between which of the nine vaccines the women received, including those from Pfizer-BioNTech, Moderna, and AstraZeneca. Though the change in cycle length is considered within the normal range of variation, the study's authors noted that it could be"alarming" to those experiencing it and contribute to hesitancy toward getting the COVID-19 vaccine.
The study did not determine what physical reasons could explain the changes but underscored the need for additional research into the issue and future studies looking at the link between vaccination and menstrual cycle changes, such as associated symptoms like bleeding.Researchers looked at data from three consecutive menstrual cycles before vaccination and at least one cycle after from 19,622 women, including 14,936 who were vaccinated and 4,686 who were not.