A U.S. National Institutes of Health study found that COVID-19 vaccines lengthened women’s menstrual cycles. The study, published in a January edition of Obstetrics & Gynecology, compared menstrual cycle lengths between women vaccinated and unvaccinated against COVID-19. “For vaccinated individuals, data was from three consecutive cycles before vaccination and from three more consecutive cycles, including the cycle or cycles in which vaccination took place. For unvaccinated individuals, data was collected for six consecutive cycles,” reads a summary of the study. Of the 3,959 individuals analyzed, 2,403 were vaccinated, with most receiving either a Pfizer or Moderna jab, and 1,556 were unvaccinated.