Women may rest a bit easier thanks to results from a study showing that vaccination against the SARS-CoV-2 virus has almost no impact on a woman’s menstrual cycle. The issue is significant, as regular menstruation is a sign of health and fertility, and fears of disturbances might increase vaccination hesitancy as COVID-19 cases continue to surge.
, a professor of obstetrics and gynecology at Oregon Health & Science University, Portland, led a group studying prospective data on almost 24,000 menstrual cycles reported by almost 4,000 U.S. women.
The investigators found that COVID-19 vaccination was associated with a less than 1-day change in cycle length for the menstrual cycles after the first and second inoculations, compared with prevaccine cycles. Vaccination had no effect on the actual number of days menstrual bleeding lasted.
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 postvaccine cycles. The final sample included 2,403 vaccinated and 1,556 unvaccinated individuals.
In vaccinated women, the study initially found a slight average increase in cycle length after dose one of 71% of a day and 91% of a day after dose two. Following 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 from baseline.
“Coronavirus disease 2019 vaccination is associated with a small change in cycle length but not menses length,” Dr. Edelman’s group concluded 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 variations appear to resolve quickly, possibly as soon as the next cycle after vaccination and do not indicate any cause for long-term physical or reproductive health concern, according to the authors.
Reports by women on, however, have suggested that postvaccine menstrual disruptions are more common with, for example, heavier and breakthrough bleeding. But it appears such changes are temporary and resolve quickly.
“These findings are reassuring and validating,” Dr. Edelman said in an interview. On a population level, the changes indicate 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.”
According to the International Federation of Gynecologists and Obstetricians, variations in cycle length of fewer than 8 days are considered normal, said Christine Metz, PhD, a research biologist and a professor of molecular medicine at the Feinstein Institutes for Medical Research in Manhasset, N.Y. “Thus, the extra 17 hours added to the menstrual cycle length in the vaccination group in this study is well within the ‘normal’ range.”
In a group of about 1,600 menstruating women being studied at Dr. Metz’s center, some have anecdotally reported transient cycle changes post vaccination for COVID-19, including delays in menstruation onset and changes in bleeding patterns.
Exactly how vaccination might alter menstrual cycle length is not known and has not been studied with vaccination against other infections such as influenza and meningococcal disease.
“Many factors are known to affect menstrual cycle length including changes in diet, sleep, and exercise, as well as sickness, travel, and stress,” Dr. Metz said. The COVID-19 vaccines have affected people in different ways, with side effects ranging from injection-site pain to nausea, aches, fever, and fatigue. “Vaccination side effects, particularly if severe, could lead to changes in diet, exercise, and sleep, and feelings of sickness and/or stress.”
These stressors can alter hormone production and stability, as well as the body’s response to hormones such as estrogen, progesterone, follicle-stimulating hormone, luteinizing hormone, and other hormones associated with female reproduction. “Because these hormones regulate the menstrual cycle, variations in these hormones can either shorten or lengthen the cycle,” Dr. Metz explained.
More research needs to be done at the global level, according to the authors. “Questions remain about other possible changes in menstrual cycles, such as menstrual symptoms, unscheduled bleeding, and changes in the quality and quantity of menstrual bleeding.”
This research was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Institutes of Health’s Office of Research on Women’s Health. Dr. Edelman reported support from the American College of Obstetrics and Gynecology, the World Health Organization, Gynuity, and the Karolinska Institute as well as royalties from UpToDate. Other study authors reported similar relationships with not-for-profit and private-sector companies. Three coauthors are employees of Natural Cycles, a fertility tracking device that was used in the study. Dr. Metz disclosed no conflicts of interest with regard to her comments.