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COVID-19 vaccine may temporarily alter menstrual cycle length, should not be cause for concern

Research shows an average change of less than one day in overall cycle duration following vaccination, no changes to period length
Image shows a person of female birth gender getting a vaccination in a healthcare setting.
Some people may experience a slight, but temporary, change in the length of their monthly cycle following COVID-19 vaccination. (Getty Images) 

A data analysis of nearly 4,000 U.S.-based individuals who track their menstrual cycles found that some people may experience a slight, but temporary, change in the length of their monthly cycle following COVID-19 vaccination.

The study results, published online in the journal Obstetrics & Gynecology, align with public reports of menstrual irregularity after vaccination made last year.

Alison Edelman, a person with short brown hair, wearing glasses, smiling.
Alison Edelman, M.D., M.P.H. (OHSU)

According to lead investigator Alison Edelman M.D., M.P.H., the average change is less than one day in the cycle where vaccination occurred; however, in the rare instance that an individual receives two vaccine doses within the same menstrual cycle, the change in cycle length may increase to two days.

“These variances appear to resolve quickly, possibly as soon as the next cycle after vaccination,” she says.

Despite the impact to overall menstrual cycle length, the analysis did not find any changes to the length of menses, or bleeding, with vaccination.

“Our findings are reassuring. On a population level, the changes we are finding indicate no cause for concern for long-term physical or reproductive health,” says Edelman, a professor of obstetrics and gynecology in the Oregon Health & Science University School of Medicine. “However, this research is critically important. People want this information so they know what to expect when they get vaccinated. This is especially true for those individuals – including those who may be hoping to achieve, or avoid, pregnancy -- where any change in the length of a monthly cycle might be troubling.”

Edelman and the OHSU research team collaborated with partners within women’s health to analyze the cycles of nearly 4,000 US-based Natural Cycles users. All users of the FDA-cleared birth control app granted permission to use their de-identified data and researchers used this data to compare cycle differences between vaccinated and unvaccinated individuals who experience consistent, and clinically normal menstrual cycles.

“Beyond our data-based findings, what is really significant about this research is that it was prompted by the public’s interest in menstrual health,” says Edelman. “While there are still unanswered questions, like the impact on menstrual flow, it is exciting to be involved in research that allows community members to feel more informed in their decision making about getting vaccinated against COVID-19.”

Future research will review data from Natural Cycles, as well as cycle tracking application Clue to better understand any additional vaccine-related changes to the menstrual cycle and menses period, including the impact of booster and third doses, both in the United States and globally.

This research is supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Developmenta part of the National Institutes of Health (grant number NICHD- HD089957).

The research team includes: Edelman, Emily Boniface, M.P.H., Leo Han, M.D., M.P.H., and Blair Darney, Ph.D., M.P.H., Department of Obstetrics and Gynecology in the OHSU School of Medicine; Elenora Benhar, Ph.D., Carlotta Favaro, Ph.D., and Jack Pearson, Ph.D., Natural Cycles USA Corp; and, Kristen Matteson, M.D., M.P.H., Department of Obstetrics and Gynecology in the Warren Alpert Medical School of Brown University.

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