HealthDay News — COVID-19 vaccination is associated with a small and mainly transient change in menstrual cycle length, according to study findings published in BMJ Medicine.

Alison Edelman, MD, MPH, from the Oregon Health & Science University in Portland, and colleagues conducted a global cohort study of prospectively collected data for international users of the menstrual cycle tracking application, Natural Cycles. Data were included for 19,622 individuals aged 18 to 45 years with cycle lengths of 24 to 38 days and with data for at least 3 cycles before and 1 after vaccination (14,936 participants [vaccinated group]) and with at least 4 consecutive cycles (4,686 participants [unvaccinated group]).

Compared with individuals who were not vaccinated, those who were vaccinated had a less than 1-day adjusted increase in the length of their first and second vaccine cycles (0.71-day increase for first dose; 0.56-day increase for second dose). A larger adjusted difference was seen for those who received 2 doses in 1 cycle (3.70-day increase). For individuals who received 1 dose per cycle, cycle length was similar to before the vaccine at 1 cycle after vaccination; individuals who received 2 doses per cycle had a 0.85-day change compared with unvaccinated individuals. Vaccination did not affect menses length.

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“Although we do find menstrual changes after COVID-19 vaccination, these changes are small compared with normal variation and resolve in the cycle after vaccination, except in people who received both doses in 1 menstrual cycle,” the authors wrote.

Several authors disclosed financial ties to the pharmaceutical and medical technology industries.

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