NIH Study Links Covid Vaccination to Slightly Increased Menstrual Cycle Frequency

According to a big multinational study supported by the National Institutes of Health, Covid-19 immunization is associated with a modest increase in the length of a woman’s menstrual cycle, delaying the onset of bleeding by a few hours.

Dr. Diana Bianchi, director of the NIH’s Institute of Child Health and Human Development, stated that vaccination-related changes appear to be minor, transitory, and within the normal range. According to a health organization, the extended menstrual cycle, which is generally roughly a month-long, did not necessarily increase the number of days of bleeding.

According to the NIH, a change in menstrual cycle duration of eight days or less falls within the normal range of variation. The average length of the participants’ menstrual periods rose by 0.71 days, or less than 24 hours, following the first vaccination dose, and by a little over half a day after the second. In women who received both doses of vaccination during a single menstrual cycle, the length of their cycle increased by 3.91 days.

More than 1,300 women, 6.2% of vaccinated participants, and 5% of unvaccinated participants in the study had an increase of eight days or more in their menstrual cycles. Younger women with lengthier cycles prior to immunization were more likely to experience a greater period delay.

After completion of the vaccine series, the length of the menstrual cycle recovered to normal for the majority of women who received one dose per menstrual cycle and by around 20 hours for those who received both doses in a single cycle.

Nearly 20,000 participants from Canada, the United Kingdom, the United States, Europe, and other countries participated in the study. Participants were administered one of nine vaccines: Pfizer-BioNTech, Moderna, Johnson & Johnson, AstraZeneca, Covishield, Sputnik, Covaxin, Sinopharm, or Sinovac.

Changes in the duration of the menstrual cycle did not vary between vaccinations.

The researchers utilized data from the Natural Cycles fertility tracking app. The app received temperature and menstrual cycle length data from female users. Users of the application have the option to give their data for research purposes without revealing their identities.

In January, the researchers published preliminary findings suggesting a connection between Covid immunization and increased menstrual cycle length; the study published this week verified the connection. Five research organizations were awarded $1.67 million by the NIH to explore the issue.

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