28.07.2022 - 17:26 [ US Department of Health & Human Services ]

COVID-19 vaccines linked to small increase in menstrual cycle length

(January 25, 2022)

The study was funded by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and Office of Research on Women’s Health. Results appeared in Obstetrics & Gynecology on January 5, 2022.

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The team found that women who received a COVID-19 vaccine had an average increase in cycle length of nearly one day for each dose. Among women who received a two-dose vaccine, the first dose was associated with a 0.71-day increase in cycle length and the second dose with a 0.91-day increase. After adjustment for age, race and ethnicity, BMI, education, and other factors, the change in cycle length was still less than one day for each dose.

Receiving two vaccine doses within the same menstrual cycle increased the cycle length further—about two days on average. Women’s cycle lengths often fluctuate, and experts consider cycle variation of up to eight days to be normal. The longer menstrual cycles after vaccination decreased in subsequent cycles, suggesting they are likely temporary. The researchers did not find any effect of COVID-19 vaccination on the number of menstrual bleeding days.