Study in Obstetrics & Gynecology Journal

Following reports from scores of women online who said that the COVID vaccine had altered their menstrual cycles, a government-funded study has ultimately confirmed their claims.

Now, a new study published Thursday in the journal Obstetrics & Gynecology has revealed that many of those vaccinated against COVID-19 saw a slight delay — by less than one day, on average — in the onset of their menstrual periods, compared to unvaccinated women. 

However, it did not substantially affect the number of bleeding days, according to findings from more than 4,000 participants.

For the ongoing, yearlong study — announced in September and funded by a $1.67 million grant from the National Institutes of Health — scientists retrieved data from a fertility tracking app utilized by women aged 18 to 45 who were not using hormonal contraception, which typically regulates a period into a stricter schedule.

About 2,400 participants were vaccinated with either the Pfizer (55%), Moderna (35%) or Johnson & Johnson (7%) vaccines. The remaining 1,500 women were unvaccinated and included in the study as a control group. 

The average increase in women’s cycles after the first vaccine dose was 0.64-day (about 15.36 hours), and 0.79-day (about 18.96 hours) following the second dose.

Lead author Alison Edelman of the Oregon Health & Science University told Agence France-Pressethat the jab’s impact is now believed to be minimal and temporary, an outcome she called “very reassuring” — and validating for the women who reported the changes.

“It validates that there is something real here,” Dr. Hugh Taylor, the chair of the department of obstetrics, gynecology and reproductive sciences at Yale School of Medicine, told the New York Times.

Researchers believe that the change may be related to immune system response. “We know that the immune system and the reproductive system are interlinked,” said Edelman. 

According to the International Federation of Gynecology and Obstetrics, a margin of change fewer than eight days within the average 28-day cycle is considered normal — and most women in the study experienced an increase in cycle length by little more than 15 to 19 hours.

Among the vaccinated, researchers followed women beginning three consecutive cycles prior to vaccination, plus three more following the shot, while the control group was observed for six consecutive cycles, generally. 

On average, women saw an increase in their menstrual cycles by less than one full day. 

Researchers noted that women who experience the greatest change in their periods were vaccinated at the start of the follicular phase of their menstrual cycle, which begins around day six, following the menses (bleeding) stage.

“Our goal is to provide menstruating people with information, mainly as to what to expect, because I think that was the biggest issue: Nobody expected it to affect the menstrual system, because the information wasn’t being collected in the early vaccine studies,” said Diana Bianchi, director of the National Institute of Child Health and Human Development, in a statement to Washington Post’s the Lily last year.

They also hope their study will help quell vaccine hesitancy — by giving patients as much information as possible. “We were worried this was contributing to vaccine hesitancy in reproductive-age women,” said Bianchi.

Taylor told the Times that the results of the study should be encouraging to women.

“I want to make sure we dissuade people from those untrue myths out there about fertility effects,” Taylor 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.”