COVID-19 Vaccination Slightly Impacts Menstrual Cycles

COVID-19 vaccines temporarily increase menstrual cycle length but not days of menses
young woman taking notes in a small notebook
(Precision Vaccinations)

A peer-reviewed study published by the journal Obstetrics & Gynecology on January 5, 2022, found that women receiving one dose of a COVID-19 vaccine during a single menstrual cycle had an increase in cycle length of nearly one day.

And a study subgroup that received two COVID-19 vaccine doses in the same menstrual cycle (358 users) had a more significant average increase in cycle length of two days.

However, this change appears to decrease in subsequent cycles, indicating that the menstrual changes likely are temporary.

The increase in cycle length—a more extended time between bleeding—was not associated with any change in the number of days of menses (days of bleeding) in this U.S. National Institutes of Health-funded study.

'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.'

Of the 3,959 individuals in the study, 2,403 were COVID-19 vaccinated, and 1,556 were unvaccinated. In addition, data from over 23,000 menstrual cycles were included.

The study authors, led by Alison Edelman, M.D., M.P.H., of Oregon Health & Science University, Portland, noted that menstrual cycles typically vary a small amount from month to month, and the increase they saw was well within the range of normal variability. 

They added that additional research is needed to determine how COVID-19 vaccination could potentially influence other menstrual characteristics, such as associated symptoms (pain, mood changes, etc.) and bleeding characteristics (including heaviness of flow).

"It is reassuring that the study found only a small, temporary menstrual change in women," said Diana W. Bianchi, M.D., director of NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development, in a related press statement.

Dr. Bianchi added that little research has previously been conducted on how vaccines for COVID-19 or other diseases could potentially influence the menstrual cycle.

The study authors analyzed de-identified data from a fertility tracking app, Natural Cycles. Users input data on their temperature and their menstrual cycles and can consent to the use of their de-identified data for research. 

For vaccinated individuals, data were from three consecutive cycles before vaccination and three more successive cycles, including the cycle or cycles in which vaccination occurred. 

For unvaccinated individuals, data was collected for six consecutive cycles. 

Again, the researchers saw no significant change in cycle length for the unvaccinated app users.

On average, the first vaccination dose was associated with a .71-day cycle increase in cycle length and the second dose with a .91-day increase. Therefore, users vaccinated over two cycles had an increase of less than one day in each of the vaccination cycles. 

There were no changes in the number of menstrual bleeding days for the vaccinated individuals.

Most vaccinated women (U.S. residents, 18–45 years with regular cycle lengths 24–38 days) received the Pfizer and Moderna mRNA vaccines.

mRNA vaccines have been found to create a robust immune response or stressor, which could temporarily affect the hypothalamic-pituitary-ovarian axis if timed correctly.

'Our findings for individuals who received two doses in a single cycle supports this hypothesis,' wrote these researchers.

The research had more than 99% power to detect an unadjusted 1-day difference in cycle length change or 0.5-day difference in menses length change by vaccination status, at a significance level of 0.0125 (98.75% CIs), to account for multiple comparisons among the four primary outcomes: cycle and menses length for the first and second vaccine-dose cycles.

Menstrual characteristics are not static and vary month to month across an individual's lifespan. The International Federation of Gynecology and Obstetrics classifies a variation in cycle length as average if less than eight days.

This study confirmed limitations. First, it may not be generalizable to the U.S. population given the selection of Natural Cycles users (more likely to be White, college-educated, and have lower BMIs than national distributions and not using hormonal contraception).

Second, they analyzed a cohort with consistent average cycle lengths to identify associations between cycle and menses length and COVID-19 vaccination.

PrecisionVaccinations publishes fact-checked research-based vaccine news.