Alison Edelman, MD, a professor of obstetrics and gynecology at Oregon Health & Science University, led a group studying data on almost 24,000 menstrual cycles reported by nearly 4,000 U.S. women.
The investigators found that COVID-19 vaccination was linked to a less than 1-day change in menstrual cycle length after the first and second vaccine doses, compared with pre-vaccine cycles. Vaccination had no effect on the actual number of days of menstrual bleeding.
The study looked at the menstrual patterns of women aged 18-45 years with normal cycle lengths of 24-38 days for the three consecutive cycles before the first vaccine dose and for three consecutive cycles after the vaccine. The final sample included 2,403 vaccinated and 1,556 unvaccinated people.
In vaccinated women, the study at first found an average increase in cycle length after one dose of 71% of a day and 91% of a day after dose two. After adjustments, those increases dropped to 64% of a day after the first dose and 79% of a day after the second dose.
In unvaccinated women, the study looked at six cycles over a similar time period and found no significant changes.
The study was published Wednesday in Obstetrics and Gynecology.
In the rare instance that a woman received two vaccine doses within the same menstrual cycle, the change in length could increase to 2 days. These changes appear to end quickly, possibly as soon as the next cycle after vaccination, and do not show any cause for long-term physical or reproductive health concern, according to the authors.
But reports by women on social media have suggested that menstrual changes after the vaccine are more common with, for example, heavier and breakthrough bleeding. But it appears any change is temporary.
“These findings are reassuring and validating,” Edelman said in an interview.
The changes reveal no cause for concern for long-term physical or reproductive health and no reason to avoid vaccination.
“On a personal level, people want this information so they know what to expect when they get vaccinated, and not worry about a pregnancy scare or be disappointed if they were trying for pregnancy,” Edelman said.
According to the International Federation of Gynecology and Obstetrics, variations in cycle length of fewer than 8 days are considered normal, said Christine Metz, PhD, a professor of molecular medicine at the Feinstein Institutes for Medical Research in Manhasset, NY. “Thus, the extra 17 hours added to the menstrual cycle length in the vaccination group in this study is well within the ‘normal’ range.”