The length of the menstrual cycle may increase by less than one day following Covid-19 vaccination in some women, according to a new National Institutes of Health (NIH) study published in the BMJ Medicine. However, these changes are temporary.

The international retrospective study analysed 19,622 women aged 18-45 years mostly from the UK, US, Canada and Europe to study the change in the menstrual cycle including its length following Covid-19 vaccination. Majority were below 35 years of age. Out of these, the vaccinated group included 14,936 women with data available for at least three consecutive cycles before vaccination and the unvaccinated group included 4,686 women with data available for a minimum of four consecutive cycles. Data was sourced from the menstrual cycle tracking application, Natural Cycles.

Among the vaccinated participants, the length of the cycle in which they were vaccinated increased by less than one day compared with those who were not vaccinated. On average, the cycle length increased by 0.71 day after the first dose and 0.56 day after the second dose. Among those who received the two doses in one cycle, the length increased by 3.91 days. No differences in the cycle length were noted according to the type of the vaccine taken. These changes were observed more among younger women and those who had a  longer cycle length prior to taking the vaccination. The increase in cycle length was temporary except among those who received both doses in the same cycle, in whom the increased length was decreased in the next cycle, but was still more than those in the unvaccinated group. Hence, women taking the Covid-19 vaccine can be counseled about the changes that may occur in the menstrual cycle length post-vaccination. They should be reassured that these changes are small and transient in nature.

Reference

1. Edelman A., et al. Association between menstrual cycle length and COVID-19 vaccination: a global cohort. BMJ Medicine. 2022. http://dx.doi.org/10.1136/bmjmed-2022-000297