The US government-funded study said that women who were unvaccinated had a delay in their period of almost a day.
The number of days of bleeding was not affected by the research.
The effects are small and expected to be temporary, as well as being very reassuring for those who experienced changes, according to the lead author of the study.
Anti-vaccine misinformation is rampant on social media and the study can help counter it.
The slight increase in menstrual cycle length is not clinically significant. Any change of less than eight days is considered normal.
Period cycles last about 28 days, but the precise amount varies from one woman to another, as well as within an individual's lifetime. During times of stress, it can change.
The scientists analyzed data from a fertility tracking app for women who were not using hormonal contraception.
The majority of the 2,400 participants werevaccinated by Pfizer, Moderna and Johnson & Johnson.
About 1,500 unvaccinated women were also included.
Data was collected from three consecutive cycles before and three more consecutive cycles after the vaccine was administered.
For unvaccinated individuals, the data was collected for six cycles.
The first vaccine dose was associated with a 0.64 day increase in cycle length and the second vaccine dose was associated with a 0.79 day increase when compared to the unvaccinated group.
The immune system's response to the vaccine could be the reason for the change.
The reproductive system and the immune system are interdependent.
The hypothalamic-pituitary-ovarian axis is what Edelman calls the "highway of how your brain talks to your uterus, or simply the body clock."
The way this axis regulates the timing of menstrual cycles appears to be disrupted by the production of inflammatory proteins called cytokines.
The changes seem most pronounced when vaccination takes place early in the follicular phase, which starts on the first day of the menstrual period and ends when ovulation begins.
A subgroup of people who received two injections of the Pfizer or Moderna vaccines during the same cycle saw an average increase in cycle length of two days, but the effect is temporary.
The team hopes to gather more data on the subsequent cycles of women who have been vaccined to confirm a long-term return to baseline and to differentiate the effects between vaccine brands.
Agence France-Presse.
