The problem is significant because regular menstruation is a sign of health and fertility, and fear of the disorder could reduce the likelihood that people will get a vaccine while COVID-19 cases continue to grow.
Alison Edelman, MD, A professor of obstetrics and gynecology at the University of Oregon Health & Science, she led a group that studied data on nearly 24,000 menstrual cycles reported by nearly 4,000 American women.
The researchers found that vaccination against COVID-19 was associated with a change in menstrual cycle length less than 1 day after the first and second doses of the vaccine, compared with beforevaccine cycles. Vaccination had no effect on the actual number of days of menstrual bleeding.
The study examined the menstrual patterns of women aged 18-45 years with a normal cycle of 24-38 days in three consecutive cycles before the first vaccine doses and three consecutive cycles after vaccination. The final sample included 2,403 vaccinated and 1,556 unvaccinated persons.
In vaccinated women, the study initially found an average increase in cycle length after one dose of 71% days and 91% days after the second dose. After adjustment, these increases fell to 64% days after the first dose and 79% days after the second dose.
In unvaccinated women, the study looked at six cycles over a similar period of time and found no significant changes.
The study was published Wednesday in Obstetrics and Gynecology.
In the rare case that a woman received two doses of the vaccine within the same menstrual cycle, the change in length could increase to 2 days. These changes appear to end rapidly, probably after the next cycle after vaccination, and show no reason for long-term concern for physical or reproductive health, according to the authors.
But reports from women about social media suggest that menstrual changes after vaccination are more frequent with, for example, more profuse and breakthrough bleeding. But any change seems to be temporary.
“These findings are compelling and confirmatory,” Edelman said in an interview.
The changes do not reveal a cause for concern for long-term physical or reproductive health or a reason to avoid vaccination.
“On a personal level, people want this information to know what to expect when they get vaccinated, not worry about pregnancy get scared or disappointed if they try to get pregnant, ”Edelman said.
According to the International Federation of Gynecology and Obstetrics, variations in cycle lengths of less than 8 days are considered normal, said Dr. Christine Metz, a professor of molecular medicine at the Feinstein Institute for Medical Research in Manhasset, NY. “Thus, an additional 17 hours added to the length of the menstrual cycle in the group vaccinated in this study is within the ‘normal’ range.”