No clear link between Covid-19 vaccines and menstrual changes: study

Korean researchers have found no direct link between Covid-19 vaccination and menstrual changes such as abnormal uterine bleeding (AUB).

A research team led by Professors Kim Sung-eun and Lee Yoo-young from the Department of Obstetrics and Gynecology at Samsung Medical Center (SMC) analyzed menstrual data from users of the period-tracking mobile app Heymoon. The study, published in the latest issue of Obstetrics & Gynecology Science, compared menstrual patterns before and after vaccination.

Using historical data from March 2021 to December 2022, the team selected records of women aged 18 to 49 with regular cycles and analyzed three menstrual cycles before and after vaccination.

The results showed that the incidence of irregular bleeding rose from 2.03–2.19 percent before vaccination to 3.35 percent afterward. The average duration of bleeding increased by 0.43 days, and the number of women reporting heavier-than-usual bleeding tripled. Menstrual cycle length also increased by 1.39 days on average, and more users reported changes in menstrual flow. However, the length of menstruation itself remained largely unchanged (–0.01 days on average), and there were no differences based on vaccine type.

The researchers emphasized that these changes fell within the normal range.

“Most of these changes are within the normal range,” said Professor Kim. “It is not a clinically meaningful finding, and even if there is a change, it is not a reason to delay vaccination."

The study was conducted in collaboration with women’s healthcare company Happy Moonday, using de-identified and encrypted data to ensure privacy protection.