COVID-19 vaccination does lead to an average increase in menstrual cycle length, according to a new large international study.

The study funded by the National Institutes of Health (NIH) found that women experienced a longer menstrual cycle length after getting vaccinated against SARS-CoV-2.

The slight increase was less than one day, so there was no notable change to the number of days of menses (days of bleeding), the NIH said in a press release published Tuesday.

Based on the study findings, the increase was consistent across data from around 20,000 female participants in Canada, the United Kingdom and other parts of Europe and the world.

Alison Edelman, M.D., M.P.H., the study’s principal investigator spoke about the team’s findings with The Washington Post. According to her, the vaccination effects were temporary, so they did not impact the fertility of the participants.

“Now we can give people information about possibly what to expect with menstrual cycles. So I hope that’s overall really reassuring to individuals,” the professor of obstetrics and gynecology at Oregon Health & Science University added.

Edelman admitted that her team was still unsure why the vaccination affected the menstrual cycles of the participants, but she noted that the immune and reproductive systems have a connection.

Per the team’s collected data, the participants’ menstrual cycles increased by less than 24 hours after the first COVID-19 vaccine dose and by around half a day after the second dose.

Once they completed the vaccination series, their menstrual cycle mostly returned to its normal length for those who received one dose per menstrual cycle.

The researchers collected data using the fertility tracking app called Natural Cycles. Participants provided information on their temperature and menstrual cycle length via the app, according to CNBC.

The new study confirmed the findings of a previous U.S. study published in January that first linked COVID-19 vaccination with an increased menstrual cycle length.

“These findings provide additional information for counseling women on what to expect after vaccination. Changes following vaccination appear to be small, within the normal range of variation, and temporary,” said Diana Bianchi, M.D., director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development.

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