Shortly after coronavirus vaccines were rolled out about a year ago, women started reporting erratic menstrual cycles after receiving the shots.

Some said their periods were late. Others reported heavier bleeding than usual or painful bleeding. 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. The authors reported that women who were inoculated had slightly longer menstrual cycles after receiving the vaccine than those who were not vaccinated.

Their periods themselves, which came almost a day later on average, were not prolonged, however, and the effect was transient, with cycle lengths bouncing back 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 begin with a seven-day period, but the cycle would last 29 days. The cycle ends when the next period starts and would revert to 28 days within 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 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.

At the same time, he added, the changes seen in the study were not significant and appeared to be transient.

“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 for postmenopausal women who experience vaginal bleeding or spotting, whether after vaccination or not, warning that they may have a serious medical condition and should be evaluated by a physician.

One serious drawback of the study, which focused on U.S. residents, is that the sample is not nationally representative and cannot be generalized to the population at large.

The data were provided by a company called Natural Cycles that makes an app to track fertility. 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.

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 National Institutes of Health’s Office of Research on Women’s Health and N.I.C.H.D. helped fund the study, as well as related research projects at Boston University, Harvard Medical School, Johns Hopkins and Michigan State University.)

“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.

Researchers looked at records from nearly 4,000 women who had meticulously tracked their menstruation in real time, including about 2,400 who were vaccinated against the coronavirus and about 1,550 who were not. All were U.S. residents aged 18 to 45 who had logged their periods for at least six months.

For those who were vaccinated, the researchers examined the three cycles before and after the vaccine to look for changes, comparing them with a similar six-month duration in women who did not receive a vaccination.

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 unvaccinated group saw no significant changes over the six months.

Future studies using the database will examine other aspects of menstruation, such as whether periods were 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 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.

Some women who were vaccinated had cycles that were eight days longer than usual, which is considered clinically significant, Dr. Edelman said.

“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. Hormones secreted by the hypothalamus, the pituitary gland and the ovaries regulate the monthly cycle, and they can be affected by environmental factors, stressors and life changes.

(The changes observed in the study were not caused by pandemic-related conditions, the authors said, since women in the unvaccinated group were also living in 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.

The information is important, just like knowing that one may experience a headache or develop a fever after vaccination, Dr. Edelman said.

“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.”