Women see brief, small increase in menstrual cycle length after COVID-19 vaccination
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Women who received one dose of a COVID-19 vaccine during a single menstrual cycle had a temporary increase in cycle length of nearly one day compared to unvaccinated women, according to a study published in Obstetrics & Gynecology.
This increase was not associated with any change in the number of days of menses, the researchers noted, adding that menstrual cycles typically vary a small amount from month to month and that this increase was within the range of normal variability.
“It is reassuring that the study found only a small, temporary menstrual change in women,” Diana W. Bianchi, MD, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), said in a 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 said, adding that little previous research has investigated how vaccines for COVID-19 or other diseases could influence the menstrual cycle.
The researchers analyzed de-identified data from the Natural Cycles fertility tracking app, where users input data about their temperature and menstrual cycles while allowing their de-identified data to be used in research.
The study included data from 2,403 vaccinated and 1,556 unvaccinated individuals. Most of the vaccinated users received the Pfizer-BioNTech (55%) or Moderna (35%) vaccines. A small percentage (7%) received the Johnson & Johnson/Janssen vaccine.
For vaccinated individuals, data was from three consecutive cycles before vaccination and from three more consecutive cycles, including the cycle or cycles when vaccination occurred. For unvaccinated individuals, data was collected for six consecutive cycles during a similar time period.
The first vaccination dose was associated with an average 0.71-day increase in cycle length (98.75% CI, 0.47-0.94), and the second dose was associated with an average 0.91-day increase (98.75% CI, 0.63-1.19). App 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.
The unvaccinated app users saw no significant change in cycle length compared with three baseline cycles.
A subgroup of 358 app users who received two vaccine doses during the same menstrual cycle had a larger average increase in cycle length of 2.38 days (98.75% CI, 1.52-3.24). But this change appears to decrease in subsequent cycles, the researchers said, indicating that these menstrual changes likely are temporary.
The International Federation of Gynecology and Obstetrics classifies a variation in cycle length as normal if the change is less than 8 days, the researchers noted. Further research is needed to determine how COVID-19 vaccination could potentially influence symptoms such as pain and mood changes and bleeding characteristics including heaviness of flow, the researchers added.
“We understand that there have been some reports of menstrual changes following COVID vaccination, and we know that menstruation can be impacted by a variety of factors, including stress, lifestyle changes or a range of underlying conditions,” Christopher M. Zahn, MD, vice president for practice activities at ACOG, told Healio. “[This study] provides important new evidence underscoring that any impact of the COVID-19 vaccines on menstruation is both minimal and temporary.
“We continue to stress that the COVID vaccines have absolutely no impact on fertility. People should continue to feel confident in the decision to be vaccinated and, when eligible, to receive a booster, and we encourage everyone aged 5 and above to get vaccinated,” Zahn said.