Individuals with COVID-19 may experience small, temporary changes in menstrual cycle length, according to a new study.

Researchers emphasise changes are minor and typically return to normal in the next menstrual cycle. In the study, published in the journal Obstetrics & Gynecology, researchers analysed menstrual cycle data from more than 6,000 individuals in 110 countries using Clue, a menstrual and reproductive health platform.

Among unvaccinated participants reporting COVID illness, researchers found, on average, a 1.45-day increase in menstrual cycle length compared with participants' previous cycle length average.

These increases were consistent with changes reported among the cohort who had received a COVID vaccination. Researchers emphasise that in both cohorts, changes resolved in the first cycle after vaccination or illness. 

In addition, they noted that the immune and reproductive systems are known to interact with each other, so while these findings aren't surprising, they should validate the public's experiences and provide reassurance that if changes in flow occur during or after COVID infection, they are likely to be small and temporary.

"Changes to your menstrual cycle can be concerning and even frightening," said Blair Darney, Associate Professor of Obstetrics and gynaecology from Oregon Health & Science University.

"We want this research to reassure individuals that on a population level, these changes are not typically a cause for concern," Darney added.

Cycle changes are likely due to temporary, disease-related activation of the immune response, but because individuals naturally experience variations in menstrual cycle duration and bleeding patterns, researchers say that it is challenging to isolate COVID as a sole cause.

The research team also notes that the study was not focused on individuals known to be experiencing long COVID.

The physician-scientists also advise that individuals who notice prolonged changes in menstruation should seek guidance from their clinician.

Looking forward, researchers hope to learn more about the biological mechanism of these changes and will continue to leverage data from cycle-tracking apps to investigate other reported variations in menstruation following vaccination, including missed cycles, unexpected vaginal bleeding and pain.