recent study funded by the National Institute of Health found that a small portion of women who received the COVID-19 vaccine saw a temporary change in the length of their menstrual cycles.
A team of researchers led by Dr. Alison Edelman of Oregon Health & Science University observed 3,959 individuals (vaccinated 2,403; unvaccinated 1,556) and over half of the cohort (55%) received the Pfizer-BioNTech COVID-19 vaccine, 35% received Moderna and 7% received Johnson & Johnson.
Researchers analyzed data from a birth control app called Natural Cycles, cleared by the Food and Drug Administration for women to track their menstrual cycles and tell when they’re most likely to become pregnant.
Edelman said the study included women with "the most normal of normal" cycle length, averaging between 24 and 38 days. Researchers tracked vaccinated women for three cycles before the shots and the immediate three cycles after, including the months they received a dose -- and compared them to unvaccinated women.