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  • A new study finds that the COVID-19 vaccine may affect a person’s menstrual cycle.
  • The changes are extremely mild and temporary, especially when compared with the stress COVID-19 can cause on the body.
  • The vaccines also do not affect fertility.

A new study finds that COVID-19 vaccines can temporarily and slightly affect a person’s menstrual cycle, but that the changes are barely detectable.

Experts also stress the vaccines do not impact fertility.

The study, published in Obstetrics & Gynecology, found that the changes to menstrual cycles were barely detectable and showed no signs of being harmful to study participants or their levels of fertility.

The study involved nearly 4,000 individuals, of which 2,403 were vaccinated and 1,556 were not.

There was a mix of vaccines used in the group, including Pfizer-BioNTech, Moderna, and Johnson & Johnson.

What the study found is that there was a small change in menstrual cycle length, but no change in the length of the period itself.

COVID-19 vaccines were associated with a less than 1-day change in cycle length.

“The vaccine has shown some mild disruption in menstrual cycling for some women,” said Dr. Taraneh Shirazian, associate professor in the department of obstetrics and gynecology at NYU Langone Health. “This has been reported as a brief change, such as a few days in the menstrual cycle, and does not result in any long lasting changes or disruption in fertility.”

It’s too soon to tell the “why” behind this change. But the most important thing to know is that the changes are minimal, if at all, and are not harmful to fertility.

While the study does show that the changes are mild and temporary, the length of a person’s menstrual cycle is fluid. Even those who have a regular cycle down to the calendar date can have changes in their menstrual cycles over time, especially with age and changes in daily life.

The study cites the International Federation of Gynecology and Obstetrics, saying that a cycle length is still normal if a period is delayed up to 8 days.

After 6 weeks without bleeding, that period can be considered a missed one. But the study showed that no participants came close to missing a period, as the changes to the cycle were minimal.

Hearing that anything may be disruptive to a person’s menstrual cycle — no matter the length of disruption — can be a cause for concern for many people. The results of this study may increase vaccine hesitancy among some, but experts insist that the COVID-19 vaccines remain safe for everyone.

“[The results of this study] should not preclude getting the vaccine. In fact, the COVID disease will likely disrupt the menstrual cycle much more than the vaccine,” said Shirazian. “The vaccine is the best way to avoid hospitalization and death from the COVID disease. The vaccine will not disrupt your fertility.”

Missed or late periods can happen for a wide variety of reasons, and the reasons are unique to each person.

One 2015 study found that menstrual irregularities can affect 2 to 5 percent of women of childbearing age. During times of stress, that number goes up even more. These factors are varied and, often, quite common, so there is often no reason to worry if a period is a few days late.

If you are concerned at all, the best person to speak with is your doctor. Also call your doctor if you have any of the following symptoms:

  • unusually heavy bleeding
  • fever
  • severe pain
  • nausea and vomiting
  • bleeding that lasts longer than 7 days
  • bleeding after you’ve already entered menopause and have not had periods for 1 year