KEY POINTS
  • A study funded by the NIH found that Covid vaccination was linked to a temporary increase in women's menstrual cycles by an average of less than a day.
  • Nearly 20,000 people participated in the study across Canada, the U.K., the U.S., Europe and other parts of the world who received one of nine different vaccines.
  • The changes in menstrual cycle length did not differ between vaccines.
A healthcare worker administers a dose of the Pfizer-BioNTech Covid-19 vaccine at a vaccination clinic in the Peabody Institute Library in Peabody, Massachusetts, U.S., on Wednesday, Jan. 26, 2022.

Covid-19 vaccination is linked to a slight increase in the length of a women's menstrual cycle, delaying the beginning of bleeding by a few hours, according to a large international study funded by the National Institutes of Health.

Dr. Diana Bianchi, head of NIH's child health and human development institute, said the changes following vaccination appear small, temporary and within the normal range. However, the longer menstrual cycle, typically about a month long, didn't necessarily increase the number of days of bleeding, according to health agency.