COVID-19 shot linked to temporary menstrual cycle changes, study says

A new study involving nearly 4,000 people found women's menstrual cycles were slightly longer after receiving the COVID-19 vaccine compared to unvaccinated women. 

The findings, published Jan. 5 in Obstetrics & Gynecology, back up anecdotal reports from more than 100,000 people who reported unexpected menstrual cycle changes around the time they received the shot in the months after the nation's vaccination rollout last year.  

Among the participants, 2,403 were vaccinated and 1,556 were not. Researchers examined the three consecutive cycles before and after participants received their vaccine and compared them to a similar period of time among unvaccinated participants. 

Overall, the vaccine was associated with a one-day increase in cycle length, meaning there was a slightly longer time in between periods.The cycle length increase, however, was not linked to any change in the number of days participants' had their period. Among a subgroup of 358 people who received both vaccine doses during the same menstrual cycle, there was a larger average increase of two days in cycle length. The changes appeared to be short-lived, with subsequent cycles returning to their normal lengths. 

"It is reassuring that the study found only a small, temporary menstrual change in women," said Diana Bianchi, MD, director of NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development. "These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly." 

Hugh Taylor, MD, chair of the department of OB-GYN and reproductive sciences at New Haven, Conn.-based Yale School of Medicine, told The New York Times that the minor changes observed in the study aren't medically harmful. Dr. Taylor, who was not involved in the study, has had patients report irregular cycles after vaccination.

"I want to make sure we dissuade people from those untrue myths out there about fertility effects," he said. "A cycle or two where periods are thrown off may be annoying, but it's not going to be harmful in a medical way." 

The study — supported by the National Institutes of Health — used data from Natural Cycles, an app used to track fertility. Researchers said a drawback to their research is that it may not be generalizable to the U.S. population given users of the app are more likely to be white, college educated people who do not use hormonal contraception and have lower body mass indexes. Researchers plan on using the app's database for future research on whether the vaccine affects other aspects of menstruation, such as period flow.

 

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