Menses may delay after Covid vaccination, study suggests

Shortly after the arrival of coronavirus vaccines about a year ago, women began to report that their menstrual cycles became irregular after receiving the vaccine.

Some said their period was late. Others reported having bleeding that was heavier than usual or painful bleeding. Some postmenopausal women who had not had a period for years reported that they had started bleeding again.

A study published on Thursday found that, in fact, women’s menstrual cycles changed after being vaccinated against the coronavirus. According to the study authors, inoculated women had slightly longer menstrual cycles after receiving the vaccine, compared with unvaccinated women.

Menstruation took an average of a day to arrive, but it didn’t last longer than usual. And the effect was fleeting; the length of the cycles returned to normal after a month or two. For example, a woman with a 28-day menstrual cycle that starts with seven days of bleeding would still start her cycle with seven days of bleeding, but the entire cycle lasted 29 days. The menstrual cycle ends when the next period begins. A month or two after vaccination, the cycle returned to the usual 28 days.

The delay was more pronounced in women who received both doses of the vaccine in the same menstrual cycle: in their case, menstruation started two days after the normal period, according to the scientists.

Published in the journal Obstetrics & Gynecology, the study is one of the first to substantiate empirical reports of women saying their menstrual cycles had changed after they were vaccinated, said Dr. Hugh Taylor, director of the department of obstetrics, gynecology and reproductive sciences at Yale School of Medicine.

“The study supports the idea that there is something real going on,” said Taylor, who has heard reports from her own patients of irregular cycles.

But he pointed out that the changes found in the study were not significant and appear to be transient.

“We want to dissuade people from believing the false myths out there about fertility effects,” Taylor said. “One or two cycles where your period is a little different than usual can be annoying, but it won’t harm anyone medically.”

But he conveyed a different message to postmenopausal women who experience vaginal bleeding, even mild, whether they have been vaccinated or not, warning that they may have a serious medical condition and need to be evaluated by a doctor.

A serious drawback of the study, which focused on women residing in the United States, is that the sample was not nationally representative and could not be generalized to the population as a whole.

The data was provided by a company called Natural Cycles that makes an app that tracks fertility. Its users are, on average, whiter and with a higher level of education, compared to the population as a whole. In addition, they are thinner than the average American woman (body weight can affect menstruation) and do not use hormonal contraceptives.

The study results should be reassuring to women of childbearing age, said Dr. Diana Bianchi, director of the Eunice Kennedy Shriver National Institute for Child Health and Human Development (NICHD). The Office of Women’s Health Research at the National Institutes of Health and the NICHD helped fund the study, as well as related research from Boston University, Harvard Medical School, Johns Hopkins and Michigan State University.

“Your doctors may say, ‘If you have an extra day of bleeding, that’s normal, not something to worry about,'” Bianchi said.

The study was carried out by researchers at Oregon Health & Science University and Brown University’s Warren Alpert Medical School, in cooperation with researchers at Natural Cycles, whose app is used by millions of women around the world.

Unidentified data from users who agreed to have their information incorporated into the survey provided a wealth of evidence on how women’s menstrual cycles have changed during the pandemic.

The researchers analyzed information from nearly 4,000 women who carefully monitored their menstruation in real time, including 2,400 women vaccinated against the coronavirus and about 1,550 unvaccinated. All were residents of the United States, aged between 18 and 45 years, and monitored their periods for at least six months.

In the case of vaccinated women, the researchers examined the three cycles before and three after the vaccine to detect changes, comparing the information with other information, also collected over six months, from unvaccinated women.

Overall, vaccination was linked to less than a full day of change in menstrual cycle length after each of the two vaccine doses, compared to cycles prior to vaccination. The unvaccinated group did not show significant changes over the six months.

Future studies using the database will examine other aspects of menstruation, for example whether bleeding was heavier or more painful after vaccination.

The new study’s findings may not apply equally to all women. In fact, according to Dr. Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University and the study’s lead author, most of the changes in cycle length occurred in a small group of 380 vaccinated women who showed a change of at least two days in their cycles.

Some of the vaccinated women had cycles that lasted eight days longer than normal, something that is considered clinically significant, Edelman said.

“Although cycle length differed on average by less than a day from the normal level, this could be important for the individual woman, depending on her point of view and her case,” she pointed out. “The woman may be expecting a pregnancy, she may be worried about the possibility of a pregnancy, she may be wearing white pants.”

It’s not clear why the menstrual cycle might be affected by vaccination, but most women with normal periods occasionally have an unusual cycle or a missed period. Hormones released by the hypothalamus, pituitary gland and ovaries regulate the menstrual cycle and can be affected by environmental factors, stress factors and life changes.

The authors said the changes seen in the study were not caused by conditions linked to the pandemic, given that women in the unvaccinated group were also living through the pandemic.

Clara Allain translation.