COVID Vaccines Might Delay Your Period by a Day or Two

Beyond anecdotal reports, the connection between COVID-19 vaccines and menstrual cycle length has been largely a mystery until recently. Now, researchers conducted a study that found that COVID-19 vaccines may delay periods by one or two days.

Researchers analyzed the menstrual cycle data of 1,556 unvaccinated participants and 2,403 vaccinated, including:

• 55% of whom received the Pfizer-BioNTech vaccine
• 35% who received Moderna
• 7% who received Johnson & Johnson

They found that participants experienced a 0.71-day increase in menstrual cycle length following a single COVID-19 dose when compared with menstrual cycles prior to vaccination. Participants who received two doses of the COVID-19 vaccine during the same cycle experienced a change of about two days.

“The average change in length is less than one day during the menstrual cycle when the shot was given,” Alison Edelman, MD, MPH, professor of obstetrics and gynecology at the Oregon Health and Science University School of Medicine, told Verywell. “People who receive two doses of a vaccine during one menstrual cycle may have a two-day change.” 

The January study was published in the journal Obstetrics & Gynecology.

What Lead to the Delay?

Although more research is needed to understand the causal relationship between COVID-19 vaccines and menstrual cycle length, Edelman hypothesizes that menstrual changes following COVID-19 vaccination might have something to deal with the biological connection between the immune and reproductive system. Vaccine-related menstrual disturbances may be related to the body’s immune response to the vaccine.

“We know that currently, available COVID vaccines are effective at activating the immune system,” Edelman said. “The immune system creates a temporary increase in small proteins called cytokines, which can briefly impact a person’s ability to regulate the menstrual cycle, potentially resulting in temporary changes to cycle timing.”

More research is needed to decipher the relationship between the shots and the menstrual cycle as well as the effect booster shots may have as well.

What Should You Do If You’re Experiencing Period Delays? 

Cindy M.P. Duke, MD, PhD, FACOG, board-certified OB-GYN, recommends making a note of any period delays and tracking your flow using a period tracking app or diary.

“Outside of vaccination, it is not uncommon for period length to vary by a day or two during the course of your month to month cycle,” Duke told Verywell. 

Therefore, an average of less than a one-day change indicates no cause for concern and does not warrant medical attention, Edelman said.

“However, should an individual experience change to their overall cycle (first day of bleeding to the next first day of bleeding) of more than eight days or there are noticeable changes over the course of three or more months, consultation with their healthcare provider may be necessary,” Edelman explained.

Reassuring Research

Edelman hopes that the study can provide answers and validation to individuals who experienced menstrual disruption following vaccination.

“As a clinician, I can help provide them with information about what to expect with vaccination, which might include a slight variation in their cycle length, and have them prepared for this possibility so that they do not need to worry,” she said.

Overall, the vaccines are safe, effective at curbing hospitalizations and deaths, and have not been shown to impact fertility in men and women.

“If menstrual disruption is the only reason an individual is not considering COVID-19 vaccination, this research should be reassuring that potential changes are minimal, appear to be temporary—and in alignment with other research specific to fertility and pregnancy—do not appear to cause long-term health or reproductive impacts,” Edelman said.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.