Women’s Periods May Be Late After Coronavirus Vaccination, Study Suggests

After receiving coronavirus vaccines about a year ago women began reporting irregular menstrual cycles.

Some people claimed their periods were delayed. Others reported bleeding that was more intense than usual or worsening. Some postmenopausal women who hadn’t had a period in years even said they had menstruated again.

A study published on Thursday found that women’s menstrual cycles did indeed change following vaccination against the coronavirus. According to the authors, women who had been vaccinated had slightly longer periods than those who didn’t.

The effect of the delayed periods, which occurred on average one day later, was not permanent. Cycle lengths returned to normal within a few months. For example, a 28-day-old woman with seven days’ worth of bleeding would still have a seven day period, but the cycle would last for 29 days. The cycle would end when the next period begins and would return to 28 days in a month or so.

Women who received both vaccines in the same menstrual cycle were more affected by the delay. Researchers found that these women had their periods two extra days later than normal.

The study, in the journal Obstetrics & Gynecology, is one of the first to support anecdotal reports from women that their menstrual cycles were off after vaccination, said Dr. Hugh Taylor, the chair of the department of obstetrics, gynecology and reproductive sciences at Yale School of Medicine.

“It validates that there is something real here,” said Dr. Taylor, who has heard about irregular cycles from his own patients.

He also said that the changes observed in the study were not significant, and seemed to be temporary.

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

He had a different message. Postmenopausal women who experience vaginal bleeding, or spotting after vaccination, should be evaluated by a doctor.

The study, which only included U.S. residents as its population, had one serious flaw: it was not nationally representative and could not be applied to the larger population.

Natural Cycles, which makes an app to track fertility, provided the data. Its users are more likely to be white and college-educated than the U.S. population overall; they are also thinner than the average American woman — weight can affect menstruation — and do not use hormonal contraception.

The findings should be reassuring for women in their childbearing years,” said Dr. Diana Bianchi of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. (The National Institutes of Health’s Office of Research on Women’s Health and N.I.C.H.D. The study was funded by the National Institutes of Health’s Office of Research on Women’s Health and N.I.C.H.D.

“Their providers can say, ‘If you have an extra day, that is normal, it’s not something to be concerned about,’” Dr. Bianchi said.

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

De-identified data from users who consented to have their information incorporated into the research provided a trove of evidence about how women’s cycles changed during the pandemic.

Nearly 4,000 women had kept track of their menstruation in real-time. There were approximately 2,400 who had been vaccinated against the coronavirus, and 1,550 who hadn’t. All were U.S. residents between 18 and 45 who had kept track of their periods for at minimum six months.

Researchers compared the three-month periods before and after vaccination for women who had been vaccinated to see if there were any changes.

Overall, vaccination was associated with less than a full day’s change in cycle length, on average, after both vaccine doses, compared with pre-vaccine cycles. The six-month period saw no significant changes in the unvaccinated group.

Future studies with the database will also examine other aspects such as how heavy or painful periods were after vaccinations.

The findings of the new study might not be the same for all women. Indeed, much of the change in cycle length was driven by a small group of 380 vaccinated women who experienced a change of at least two days in their cycle, said Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University and the paper’s lead author.

Dr. Edelman explained that some women who were vaccinated experienced eight-day cycles, which is clinically significant.

“Though the cycle length was less than one day different at the population level, for an individual, depending on their perspective and what they’re relying on menses for, that could be a big deal,” she said. “You might be expecting a pregnancy, you might be worrying about a pregnancy, you might be wearing white pants.”

It’s not clear why the menstrual cycle might be affected by vaccination, but most women with regular periods experience an occasional unusual cycle or missed period. The hormones secreted in the hypothalamus, pituitary and ovaries regulate the monthly cycle. They can be affected if there are stressors or environmental factors.

(The authors claimed that the changes in the study weren’t caused by pandemic-related conditions because the women in unvaccinated groups were also experiencing the pandemic.

Whether other vaccines affect menstruation is not known — clinical trials of vaccines and therapeutics do not generally track menstrual data points, unless investigators are specifically testing therapeutics as contraceptives or fertility enhancers, or they want to rule out pregnancy.

“We’re hoping this experience will encourage vaccine manufacturers and clinical trials of therapeutics to ask questions about the menstrual cycle, the same way you’d include other vital signs,” Dr. Bianchi said.

Dr. Edelman said that the information is just as important as knowing that one could experience a headache after vaccinations.

“Individuals who menstruate spend a week out of every month, sometimes more, having to deal with menstruation,” Dr. Edelman said. “If you add up the time over 40 years, it’s practically ten years of menstruation.”