The increase, though less than one day, has been consistent across data from nearly 20,000 people in Canada, the United Kingdom and other parts of Europe and the world. The study was originally published in the British Medical Journal.
"These findings provide additional information for counseling women on what to expect after vaccination," said Diana Bianchi, M.D., director of the National Institutes of Heath's Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) in a release. "Changes following vaccination appear to be small, within the normal range of variation, and temporary."
Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University led the study. Speaking with The Washington Post, she said the effects her team discovered were temporary, and that no indication was found that they had any bearing on fertility.
"Now we can give people information about possibly what to expect with menstrual cycles," Edelman said. "So I hope that's overall really reassuring to individuals."
Although Edelman and her team still don't know exactly why the vaccines appear to affect menstrual cycles, she said the immune and reproductive systems are linked. She emphasized the parameters of her own study to the outlet, acknowledging that her study did not include individuals who were on birth control. It also only used data from people who had regular cycles before getting vaccinated and fell between the ages 18 and 45.
The study was funded by the NIH and National Institute of Child Health and Human Development as part of $1.67 million awarded to five institutions to explore potential links between COVID-19 vaccination and menstrual changes.
Never miss a story — sign up for PEOPLE's free daily newsletter to stay up to date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.
The study found that participants who received any of nine different vaccines against the virus had an increased number of days on their period, but the increase resolved in cycles following vaccinations. "De-identified" data from the period-tracking app Natural Cycles was analyzed in both studies.
Of the thousands of participants, 14,936 were vaccinated, while 4,686 were not. Researchers were able to analyze data that app users inputted about their menstrual cycles every month, observing three cycles before vaccination and one cycle after. They then compared the information to the unvaccinated group.
Data revealed that vaccinated people got their periods, on average, 0.71 days late after the first dose of a vaccine. People who had two shots within one menstrual cycle experienced the greatest amount of disruption, increasing cycles by four days, with a 13 percent delay of eight days or more.
The authors called for future studies to be done on the aspects of vaccination-linked changes to menstrual cycles. They said further investigation is required into unexpected bleeding as well as changes in flow and levels of pain.
After anecdotal reports popped up on social media from people reporting changes to their menstrual cycles, the NIH-funded research into possible links between the two, with the first study published in April in the journal Obstetrics and Gynecology.
Researchers also pulled that data from the app Natural Cycles, which helps people to track their menstrual cycles and found that the vaccines were associated with a change of less than one day in the length of the menstrual cycle or a shift in when they began — "no population-level clinically meaningful change," they wrote.
As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from the CDC, WHO and local public health departments.