A large international study published by the British Medical Journal reports an average increase in menstrual cycle length of less than one day.
The increase was not associated with any change in the number of ays of menses (days of bleeding). Funded by the National Institutes of Health, the study included data from around 20,000 people from the US, UK, Europe, Canada and other parts of the world.
Alison Edelman, M.D.,M.P.H., of Oregon Health & Science University in Portland, was principal investigator and found the the increase resolved in the cycle following vaccination.
A change in cycle length of less than eight days in considered within the normal range of variation. Although small menstrual changes may not be meaningful to health care professionals, the study authors said that any changes in bodily function linked to fertility may be alarming to those experiencing it.
This could lead to vaccine hesitancy, the authors relayed. Researchers analysed data from the fertility tracking app Natural Cycles.
A total of 19,622 individuals participated, with 14,936 being vaccinated and 4,686 unvaccinated. Data on at least 3 consecutive cycles before vaccination and at least one cycle after.
On average, vaccinated people experienced an increase of less than one day in each cycle. Those who had both doses in a single cycle had a 3.91 day increase in cycle length. After vaccination, cycle length increased by 0.2 days for 1 dose per cycle and 0.85 days for 2 doses in 1 cycle.
Of the total, 1,342 participants experienced a change in cycle length of 8 or more days, comprising 6.2% of vaccinated and 5.0% of unvaccinated individuals.
In conclusion, ‘Covid-19 vaccination is associated with a small and likely to be temporary change in menstrual cycle length but no change in menses length.’