Shortly after coronavirus vaccines were rolled out about a year ago, women began reporting irregular menstrual cycles after receiving the injections.
Some said their periods were late. Others reported heavier than normal bleeding or painful bleeding. Some postmenopausal women who haven’t had periods for years even said they had their period again.
A study published Thursday found that women’s menstrual cycles did indeed change after being vaccinated against the coronavirus. The authors reported that women who had been vaccinated had slightly longer menstrual cycles after receiving the vaccine than those who had not been vaccinated.
However, their periods themselves, which came almost a day later on average, were not prolonged and the effect was transient, with cycle length returning to normal within one or two months. For example, someone with a 28-day menstrual cycle that starts with seven days of bleeding would still start with a seven-day period, but the cycle would last 29 days. The cycle ends when the next menstrual period begins and should return to 28 days in a month or two.
The delay was more pronounced in women who received both vaccine doses during the same menstrual cycle. These women had their periods two days later than usual, the researchers found.
The study, in the journal Obstetrics & Gynecology, is one of the first to support anecdotal reports from women that their menstrual cycles had dropped after vaccination, said Dr. Hugh Taylor, the chair of the obstetrics, gynecology and reproductive sciences division at Yale School of Medicine.
“It confirms that there is something real here,” said Dr. Taylor, who has heard about irregular cycles from his own patients.
At the same time, he added that the changes observed in the study were insignificant and seemed transient.
“I want to make sure we keep people off of those false myths out there about fertility effects,” said Dr. Taylor. “A cycle or two where menstruation drops can be annoying, but it won’t be harmful in a medical way.”
He had a different message for postmenopausal women who experience vaginal bleeding or spotting, with or without vaccination, warning that they may have a serious medical condition and should be evaluated by a doctor.
A serious drawback of the study, which focused on US residents, is that the sample is not nationally representative and cannot be generalized to the entire population.
The data was provided by a company called Natural Cycles that makes an app to track fertility. Its users are more likely to be white and highly educated than the general American population; they’re also thinner than the average American woman — weight can affect menstruation — and don’t use hormonal birth control.
For women in their childbearing years, the findings should be reassuring, said Dr. Diana Bianchi, the director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. (The Office of Research on Women’s Health and the National Institutes of Health’s NICHD helped fund the study, as well as related research projects at Boston University, Harvard Medical School, Johns Hopkins, and Michigan State University.)
“Their health care providers may say, ‘If you have an extra day, that’s normal, it’s nothing to worry about,'” said Dr. bianchi.
The study was conducted by researchers at Oregon Health & Science University and Brown University’s Warren Alpert Medical School, in collaboration with researchers at Natural Cycles, whose app is used by millions of women around the world.
Anonymized data from users who agreed to have their information included in the study provided a wealth of evidence about how women’s cycles changed during the pandemic.
Researchers looked at data from nearly 4,000 women who closely tracked their periods in real time, including about 2,400 who had been vaccinated against the coronavirus and about 1,550 who had not. All were US residents aged 18 to 45 who had their periods recorded for at least six months.
For those who had been vaccinated, the researchers examined the three cycles before and after the vaccine to look for changes, and compared them to a similar six-month duration in women who did not receive the vaccination.
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Overall, vaccination was associated with an average of less than a full day change in cycle length after both vaccine doses, compared with pre-vaccination cycles. The unvaccinated group saw no significant changes over the six months.
Future studies using the database will examine other aspects of menstruation, such as whether menstruation was heavier or more painful after vaccination.
The findings of the new study may not apply equally to all women. Indeed, much of the change in cycle length was caused by a small group of 380 vaccinated women who experienced at least a two-day change in their cycle, said Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University and the paper’s lead author.
Some women who were vaccinated had cycles that were eight days longer than normal, which is considered clinically significant, said Dr. Nobleman.
“Although cycle length differed by less than a day at the population level, for an individual, depending on their perspective and what they rely on menstruation for, that can be a major problem,” she said. “Maybe you’re expecting a pregnancy, maybe you’re worried about a pregnancy, maybe you’re wearing white pants.”
It’s not clear why the menstrual cycle can be affected by vaccination, but most women with regular periods occasionally have an unusual cycle or a missed period. Hormones secreted by the hypothalamus, pituitary gland and ovaries regulate the monthly cycle and can be influenced by environmental factors, stressors and life changes.
(The changes seen in the study were not caused by pandemic conditions, the authors said, as women in the unvaccinated group also lived in the pandemic.)
Whether other vaccines affect menstruation isn’t known — clinical trials of vaccines and therapies generally don’t track menstrual data unless researchers specifically test therapies like contraceptives or fertility enhancers, or want to rule out pregnancy.
“We hope this experience will encourage vaccine manufacturers and clinical trials of therapies to ask questions about the menstrual cycle in the same way you would record other vital signs,” said Dr. bianchi.
The information is important, as is knowing you could get a headache or run a fever after being vaccinated, said Dr. Nobleman.
“Individuals who menstruate have a week, sometimes more, each month to deal with menstruation,” said Dr. Nobleman. “If you add up the time over 40 years, it’s almost ten years of menstruation.”