National Institutes of Health: COVID-19 Vaccines Linked to Small Increase in Menstrual Cycle Length

The study was funded by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and Office of Research on Women’s Health. Results appeared in Obstetrics & Gynecology on January 5, 2022.

The team analyzed data from nearly 4,000 women, ages 18 to 45, collected through a fertility tracking app. Participants agreed to have their data used for research purposes. All the women had normal cycle lengths, from 24 — 38 days. Data was compared for three menstrual cycles before the first vaccine dose to the three cycles afterward. Among unvaccinated women, data was collected for six consecutive cycles. Most of the vaccinated participants (55%) received the Pfizer-BioNTech vaccine, 35% received the Moderna vaccine, and 7% received the Johnson & Johnson/Janssen one.one.

The team found that women who received a COVID-19 vaccine had an average increase in cycle length of nearly one day for each dose. Among women who received a two-dose vaccine, the first dose was associated with a 0.71 — day increase in cycle length and the second dose with a 0.91 — day increase. After adjustment for age, race and ethnicity, BMI, education, and other factors, the change in cycle length was still less than one day for each dose.

Receiving two vaccine doses within the same menstrual cycle increased the cycle length further — about two days on average. Women’s cycle lengths often fluctuate, and experts consider cycle variation of up to eight days to be normal. The longer menstrual cycles after vaccination decreased in subsequent cycles, suggesting they are likely temporary. The researchers did not find any effect of COVID-19 vaccination on the number of menstrual bleeding days.

“It is reassuring that the study found only a small, temporary menstrual change in women,” says NICHD Director Dr. Diana Bianchi, “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.”