BETHESDA, Md. (StudyFinds.org) — COVID vaccinations may temporarily lengthen a woman’s menstrual cycle, a new study reveals. Researchers found that women receiving one dose of a coronavirus vaccine during their menstrual cycle saw their cycle increase by up to one day in comparison to unvaccinated women.

The study, funded by the National Institutes of Health (NIH), finds the changes in cycles affects the time between bleeding, but not the period of bleeding itself.

Still within the ‘normal’ range?

Study authors, led by Dr. Alison Edelman from the Oregon Health & Science University in Portland, confirmed that it is not uncommon for menstrual cycles to vary from month to month and the increase they found was “well within the range of normal” variability. Edelman adds that further research is necessary to determine how COVID vaccines could influence other menstrual characteristics such as pain, mood changes and the heaviness of flow.

“It is reassuring that the study found only a small, temporary menstrual change in women,” says Dr. Diana Bianchi, the director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a media release.

“These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly.”

Bianchi notes that there has been little research on how COVID vaccines or vaccines for other diseases could potentially influence the menstrual cycle. In a bid to understand why the changes occur, the team analyzed data from the fertility tracking app Natural Cycles.

To get an accurate reading, users have to input data on their temperature and menstrual cycles. For vaccinated women, researchers looked at data from three consecutive cycles before the patients received the vaccine and from three more consecutive cycles after — including the cycle or cycles in which their vaccination took place. For unvaccinated participants, researchers collected their data from six consecutive cycles.

Menstrual cycle length can last up to 2 days longer in some cases

Of the 3,959 individuals in the study, 2,403 were vaccinated and 1,556 were unvaccinated. Most of the vaccinated app users received either the Pfizer or Moderna vaccines. On average, the first vaccination dose displayed a connection to a 0.71-day increase in a woman’s cycle length. A second COVID vaccine dose increased cycle length by 0.91 days.

There were no changes in the number of menstrual bleeding days for the vaccinated patients and the researchers did not detect any significant change in the cycle length for unvaccinated app users. However, 358 app users who received two doses of the coronavirus vaccine during the same menstrual cycle experienced a noticeably larger average increase in their cycle length — lasting up to two days longer.

According to the researchers, this change appeared to decrease in the following cycles, which suggests that the menstrual changes are temporary and will go away shortly after vaccination. Bianchi and the team adds that the International Federation of Gynecology and Obstetrics classifies a change in cycle length as “normal” if the change is less than eight days.

The study is published in the journal Obstetrics & Gynecology.

South West News Service writer Georgia Lambert contributed to this report.