Women who received the coronavirus vaccine experienced almost a day longer menstrual cycle than usual, a new study reveals.

The findings, published in the journal Obstetrics & Genecology, validate claims from thousands of women who reported erratic changes in their cycle after getting the shot.

A team of scientists led by Oregon Health and Science University analyzed cycles of nearly 4,000 women through a fertility tracking app - some of which were vaccinated and others were unvaccinated.

The average increase in women’s cycles after the first vaccine dose was 0.64-day (about 15.36 hours), and 0.79-day (about 18.96 hours) following the second dose.

However, a subgroup of app users who received two vaccine doses in the same menstrual cycle (358 users) had a larger average increase in cycle length of two days.  

 

 

A team of scientists led by Oregon Health and Science University analyzed cycles of nearly 4,000 women through a fertility tracking app - some of which were vaccinated and others were unvaccinated

Period cycles generally last about 28 days, but the precise amount varies from one woman to another, as well as within an individual's lifetime. It can also change during times of stress.

The findings, however, are preliminary, as this study is set to run a full year.

Diana W. Bianchi, M.D., director of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), said in a statement: ‘It is reassuring that the study found only a small, temporary menstrual change in women.

The universities include Boston University, Harvard Medical School, Johns Hopkins University, Michigan State University and Oregon Health and Science University.

‘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.’

For their study, the scientists analyzed anonymized data from a fertility tracking app, Natural Cycles, among women aged 18 to 45 who were not using hormonal contraception. 

Researchers collected data from three consecutive cycles before getting the vaccine and three more after. DailyMail

According to the Journal (Obstetric&Gynecology):

Our primary outcome was the within-individual change in cycle length (in days) from the three-cycle prevaccination average to the initial vaccination cycle. For vaccinated individuals, cycle four was the first vaccine-dose cycle; the cycle of the second dose varied based on when the second vaccine dose occurred (cycle four, five, or six). For the unvaccinated cohort, we designated cycle four as the artificial first vaccine-dose cycle and cycle five as the artificial second-dose cycle; cycles one, two, and three were considered the equivalent of prevaccination cycles. Secondary outcomes were the same within-individual change in cycle length for the second vaccination cycle and corresponding changes in menses length for the first and second vaccine-dose cycles. We also examined the proportion of participating individuals who experienced a clinically significant change in cycle length (8 days or more).