Well-Being Prevention & Cures

COVID vaccination temporarily lengthens menstrual cycle

A large global study confirms a link between COVID-19 vaccination and a temporary increase of the menstrual cycle by less than a day.
woman wearing a face mask getting a shot in her left arm

Story at a glance

  • Many people have reported changes in their menstrual cycle after getting vaccinated for COVID-19.

  • A study from earlier this year reported data to suggest an average increase of one day. 

  • Globally, a larger study has confirmed a link between coronavirus vaccination and a temporary increase in length of menstrual cycles.

COVID-19 vaccination temporarily lengthens the menstrual cycle by less than a day on average, according a new study published Tuesday in BMJ Medicine

A previous study from the same team published in January found that a coronavirus vaccine slightly lengthened menstrual cycles among nearly 4,000 people in the U.S. They found that menstrual cycles temporarily increased by 0.71 day on average after the first shot and 0.56 day after the second shot.  

The latest study is part of a global effort and includes a cohort of more than 19,600 people. Nearly 15,000 people in the study were vaccinated and less than 4,700 were not. 

The data confirms that vaccination can increase the length of menstrual cycles by about less than a day, although number of days of menses (vaginal bleeding) is unaffected.

For people who received both doses within the same menstrual cycle, they saw an increase in their cycle length by 3.91 days.  

After vaccination, cycle length trended back towards baseline: 0.02 increase for people who had both doses not in the same cycle and 0.85 days for people who had both doses within the same cycle. 

A subset of individuals, about 1,300 people, experienced an increase in their cycles by eight or more days. 

“These findings provide additional information for counseling women on what to expect after vaccination,” said physician Diana Bianchi, director of the National Institute of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), in a press release. “Changes following vaccination appear to be small, within the normal range of variation, and temporary.” 

Authors say further studies are needed to investigate other aspects of the menstrual cycle and how vaccination may affect it. These may include unexpected vaginal bleeding and menstrual flow, as well as pain. The mechanism for why these changes are occurring is also unknown and more studies will be necessary.