Out of the 3959 who were a part of the study, 2403 of the individuals were vaccinated and 1556 were unvaccinated.
Women who receive 1 dose of a COVID-19 vaccine during a single menstrual cycle had an increase in cycle length of nearly 1 day compared to unvaccinated women, according to a National Institutes of Health (NIH) study. Additionally, the researchers found that the increase in cycle length was not related to any change in the number of days of bleeding.
According to study author Alison Edelman, MD, MPH, menstrual cycles vary a small amount from month to month and the increase they saw was within the range of normal variability. The study authors added that future research is needed to determine how COVID-19 vaccination could potentially influence other menstrual characteristics, such as associated symptoms and characteristics of bleeding.
“It is reassuring that the study found only a small, temporary menstrual change in women,” said Diana W. Bianchi, MD, director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, in the press 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 added that little research has previously been conducted on how vaccines for COVID-19 or vaccines for other diseases could potentially influence the menstrual cycle.
The study authors analyzed de-identified data from a fertility tracking app called Natural Cycles. Users can input data on their temperature and their menstrual cycles and can consent to the use of their de-identified data for research.
For those who are vaccinated, data were from 3 consecutive cycles before vaccination and from 3 more consecutive cycles, including the cycle or cycles in which vaccination took place. For those who are unvaccinated, data were collected for 6 consecutive cycles. Out of the 3959 who were a part of the study, 2403 of the individuals were vaccinated and 1556 were unvaccinated.
The first vaccination dose was associated with a .71-day cycle increase in cycle length and the second dose with a .91-day increase on average. Users who were vaccinated over 2 cycles had an increase of less than 1 day in each of the vaccination cycles. Additionally, there were no changes in the number of menstrual bleeding days for the vaccinated individuals, with no significant change in cycle length for the unvaccinated app users, according to the press release.
Further, a subgroup of app users who received 2 vaccine doses in the same menstrual cycle had a larger average increase in cycle length of 2 days. This change seems to appear to decrease in subsequent cycles, indicating that the menstrual changes likely are temporary. The study authors added that the International Federation of Gynecology and Obstetrics classified a variation in cycle length as normal if the change is less than 8 days.
COVID-19 vaccination associated with a small, temporary increase in menstrual cycle length, suggests NIH-funded study. NIH. January 6, 2022. Accessed January 7, 2022. https://www.nih.gov/news-events/news-releases/covid-19-vaccination-associated-small-temporary-increase-menstrual-cycle-length-suggests-nih-funded-study