Researchers supported by NICHD have found that COVID-19 vaccination is associated with a small, temporary increase in menstrual cycle length. The research found no association between vaccination and an increase in the number of days of bleeding.

Researchers supported by NICHD have found that COVID-19 vaccination is associated with a small, temporary increase in menstrual cycle length. The research found no association between vaccination and an increase in the number of days of bleeding.

Update: October 5, 2022

In an international study of data from nearly 20,000 people who received any of nine different COVID-19 vaccines, researchers have confirmed a link between COVID-19 vaccination and an average increase in menstrual cycle length of less than one day. This increase was not associated with a change in the number of days of bleeding, and for most study participants, the increase resolved in the menstrual cycle following vaccination.

The study was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and NIH’s Office of Research on Women’s Health. These results confirm the findings of a previous U.S. study conducted by the same principal researcher and supported by NICHD (see below). The results of the international study were first published in September 2022.

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Page last updated: October 5, 2022