Covid vaccinations could add an extra day to the menstrual cycle, a study has found.
Those who received one dose of a vaccine during their menstrual cycle noticed that their cycle had lengthened by up to a day, compared to people who had not been vaccinated.
The study, funded by the National Institutes of Health (NIH) and led by Dr Alison Edelman from the Oregon Health and Science University in Portland, said the increase was “well within the range of normal” variability.
The authors of the study also pointed out that it is not uncommon for menstrual cycles to vary from month to month.
Dr Edelman added that further research would be needed to determine how the jabs against Covid could influence other menstrual characteristics like pain, mood changes, and the heaviness of flow.
Dr Diana Bianchi, the director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), said: “It is reassuring that the study found only a small, temporary menstrual change in women.
“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.”
Dr Bianchi went on to explain that little research has been conducted on how Covid vaccines or vaccines against other diseases could potentially influence the menstrual cycle.
In a bid to understand why the changes happened, the study’s authors analysed data from the fertility tracking app, Natural Cycles.
To get an accurate reading, users are required to input data on their temperature and menstrual cycles and can consent to the use of their de-identified data for research.
For vaccinated people, the data was taken from three consecutive cycles before they were vaccinated and from three more consecutive cycles, including the cycle or cycles in which their vaccination took place.
For unvaccinated people, their data was collected for six consecutive cycles.
Of the 3,959 individuals in the study, 2,403 were vaccinated and 1,556 were unvaccinated.
Most of the vaccinated app users received the Pfizer and Moderna vaccines.
On average, the first vaccination dose was associated with a 0.71-day cycle increase in cycle length and the second dose with a 0.91-day increase.
According to the experts, this meant that the users who were vaccinated over two cycles had an increase of less than one day in each of their vaccination cycles.
There were no changes in the number of menstrual bleeding days for the vaccinated people and the researchers saw no significant change in the cycle length for the unvaccinated app users.
However, 358 app users who received two doses of the vaccines in the same menstrual cycle, had experienced a noticeably larger average increase in their cycle length by two days.
According to the researchers, this change appeared to decrease in the following cycles, which suggested that the menstrual changes are temporary and will go away in a short space of time.
Dr Bianchi and her team added that the International Federation of Gynaecology and Obstetrics classifies a change in cycle length as ‘normal’ if the change is less than eight days.
The study was published in the journal Obstetrics and Gynaecology.