Study links Covid-19 vaccination to menstrual cycle length

A global study published on September 27 showed that Covid-19 vaccines lead to irregularities in menstrual cycles. Changes in women’s periods have been heavily reported, ranging from missed cycles to heavier or lighter flow, and spotting.

Published in the British Medical Journal (BMJ), a study gathered 19.622 women between the age of 18 and 45 with a normal prevaccination menstrual cycle between 24 and 38 days to analyze their data and look for changes and irregularities.

Around 14,936 participants were vaccinated and 4,686 were not vaccinated.

It was hard to conduct this study because clinical trials of Covid-19 vaccines did not collect outcomes related to menstrual cycles.

Unlike a previous study by the same researchers linking the vaccine to the menstrual cycle solely based on data from the US, this recent study collected data using the digital fertility awareness application, ‘Natural Cycles’, for a bigger reach considering that the application is used globally.

This has allowed the researchers to provide general results.

They analyzed the data of many cycles before and after the vaccination.

The researchers have discovered that individuals who were vaccinated had less than a one-day increase in the length of their first and second vaccine cycles, compared with individuals who were not vaccinated.

The time difference was slightly larger among groups who got two vaccine doses. They had a 0.76-day increase in cycle length.

Overall, the results showed that a menstrual cycle length is not affected by the vaccine’s mechanism of work (mRNA, adenovirus vector, or inactivated virus), meaning that all Covid-19 vaccines can be linked to menstrual cycle changes.

The increase in cycle length for both first and second vaccine dose cycles appears to be largely driven by individuals who received both vaccine doses within a single cycle.

These results are consistent with the findings of previous studies but the recent study offers a larger more geographically diverse analysis linking Covid-19 vaccines to menstrual cycles.

“Changes to routine bodily functions can be alarming, and cause fear and anxiety,” said Alison Edelman, a leading scientist in the research.

“We want to emphasize that the menstrual cycle changes at a population level are small and typically resolve in the cycle after vaccination. Hopefully, this knowledge provides clarity about what people can expect after getting vaccinated.”

The researchers have called for future studies to assess other aspects of changes in menstrual cycles, such as unexpected vaginal bleeding, menstrual flow, and pain.