Why do these changes occur?

Some level of variation in menstruation — the number of days you bleed, the heaviness of your flow and your cycle length — is normal.

“Our menstrual cycles are not perfect clocks,” said Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University who has also studied the impact of Covid-19 vaccines on menstruation.

Hormones secreted by the hypothalamus, the pituitary gland and the ovaries regulate the monthly cycle, and they can be affected by both internal and external factors. Stress and illness, weight loss or weight gain, calorie restriction and intense exercise can all change typical patterns of menstruation.

The endometrium, which lines the uterus and is shed during menstruation, has also been linked to the immune system. Because of the role it plays in the remodeling of uterine tissue and offering protection against pathogens, it is possible that when vaccines activate the immune system, which is what they should be doing, they also somehow trigger downstream effects in the endometrium, causing a disturbance in your menstrual cycle, Dr. Edelman said. And some individuals may be more sensitive to immune or hormone changes in their body.

In her research, Dr. Edelman found that some women’s periods came a day or two later than usual after they got vaccinated against coronavirus. But the changes were temporary — menstruation tended to return to normal after one or two cycles.

What to do if you notice menstrual irregularities after the Covid vaccine

If you experience any new or unusual patterns of bleeding, take note of it. The menstrual cycle can be thought of as another vital sign, just like your body temperature or blood pressure, that provides clues about your health, said Dr. Jennifer Kawwass, a reproductive endocrinologist at Emory University, who was not involved in the study.

That being said, subtle variation in your menstrual cycle, if you have regular periods, should not be a cause for concern and does not require that you change anything you would normally do, Dr. Kawwass said.

Clinical trials and other studies have already established that the Covid-19 vaccines are safe and effective and are unlikely to impact fertility in the long term.

Should you get vaccinated at a certain time in your cycle?

Experts agree that the chaos Covid-19 can cause throughout your body, including potential lingering effects, is far worse than any side effects caused by vaccination against the disease.

People who have previously gotten a fever after a shot may plan their next dose on a day when they will not have to go in to work, Dr. Edelman said. But you should not let temporary menstrual changes prevent you from getting fully vaccinated or boosted. Since cases are on the rise again, delaying vaccination for two weeks or longer may significantly increase your risk of getting Covid-19, she said.

Still, it’s important to track your body’s response to vaccination, and public health officials should acknowledge concerns about menstrual cycle variations in addition to warning people of the risk of getting Covid-19, said Keisha Ray, a bioethics expert at McGovern Medical School at UTHealth Houston.

The increased transparency around menstrual changes or other side effects of vaccination could also have another benefit: reducing people’s vaccine hesitancy.

“We’re trying to be truthful. We’re trying to validate people’s lived experiences,” said Dr. Lee. In turn, she hopes that the new research will help improve conversations around people’s health and lead to more inclusive clinical trials in the future.