WashingtonThe COVID-19 vaccine increased the length of the menstrual cycle by almost a day in women who received a single dose of the vaccine, according to the findings of a new study.
An increase in cycle length—a longer time between bleeding—was not associated with any change in the number of menstrual days (bleeding days). The study is published in the ‘Journal of Obstetrics and Gynecology’.
MPH from Oregon Health and Science University, Portland. The authors, led by Alison Edelman, noted that menstrual cycles generally varied by a small amount from month to month, and that the increase they observed was within the range of normal variability.
He said additional research is needed to determine how COVID-19 vaccination could potentially affect other menstrual characteristics, such as associated symptoms (pain, mood changes, etc.) and bleeding characteristics ( including the heaviness of the flow).
Bianchi, director of the NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIHDD), said, “It is reassuring that the study found only a small, temporary menstrual change in the women.”
“These results provide for the first time an opportunity to advise women on what to expect from COVID-19 vaccination so that they can plan accordingly,” he said.
Dr Bianchi said that there has been little research before on how vaccines for COVID-19 or vaccines for other diseases could potentially affect the menstrual cycle.
The study authors analyzed de-identified data from the fertility tracking app, Natural Cycles. Users input data on their temperature and their menstrual cycle and may consent to the use of their non-identified data for research. For vaccinated individuals, there were data from three consecutive cycles including three consecutive cycles before vaccination and the cycle or cycle of vaccination.
For unrelated individuals, data were collected for six consecutive cycles. Of the 3,959 individuals involved in the study, 2,403 were vaccinated and 1,556 were not vaccinated.
Most vaccination users received Pfizer and Moderna Vaccines. 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 who were vaccinated in two cycles increased by less than a day in each vaccination cycle. There was no change in the number of days of menstrual bleeding for vaccinated individuals. The researchers saw no significant change in cycle length for unvaccinated app users.
There was a large average increase in cycle length of two days in a subgroup of app users who received two vaccine doses in the same menstrual cycle (358 users). However, this change appeared to decrease in later cycles, indicating that the change in menstruation is likely temporary.
The authors said the International Federation of Gynecology and Obstetrics classifies a change in cycle length as normal if the change is less than eight days.
First published:January 13, 2022, 10:05 pm