One of the first studies to determine if COVID-19 vaccination could affect the menstrual cycle in women found a small and temporary change.
The study, published Wednesday, tracked about 4,000 women in the US for six menstrual periods, and on average, the next post-vaccination period started about a day later than usual. But after the COVID-19 vaccination, the number of days of menstrual bleeding has not changed.
“This is incredibly reassuring,” said Dr. Alison Edelman of Oregon Health and Science University, who led the study, and said it’s important to tell women what to expect.
Some women report irregular periods or other menstrual changes after vaccinations. The National Institutes of Health is funding research to see if there is any link.
Edelman’s team analyzed data from a FDA-approved birth control app called Natural Cycles so that women can track their menstrual cycles and determine when they are most likely to get pregnant.
Menstrual cycles are calculated from the first day of one period to the first day of the next. Small fluctuations from month to month are normal, and stress, diet, and even exercise can cause temporary changes.
Edelman said the study included women with the “most normal to normal” cycle length, an average of 24 to 38 days. The researchers tracked vaccinated women for three cycles before and three immediately after vaccinations, including the months they received the dose, and compared them with unvaccinated women. The app prompted women to enter vaccine information.
A subgroup of 358 women who received both doses of the vaccine in one menstrual cycle saw a slightly larger change in the length of the next cycle, averaging two days. About 10% of them changed for eight days or more, but subsequently returned to normal, the researchers said in the journal Obstetrics and Gynecology.
Edelman said one theory is that when the immune system picks up speed at certain points in the cycle, “our biological clock, or whatever controls the menstrual cycle, may hiccup.”
She is planning additional research to see if there are changes in the severity of menstrual bleeding or if women with irregular periods respond differently.
The results provide “important new evidence highlighting that any menstrual exposure to the COVID vaccine is both minimal and temporary,” said Dr. Christopher Zahn of the American College of Obstetricians and Gynecologists in a statement.