question: Many people began sharing that they experienced unexpected menstrual bleeding after SARS-CoV-2 inoculation. This emerging phenomenon was undeniable yet understudied. We investigated menstrual bleeding patterns among currently and formerly menstruating people, with a research design based off our expectations that these bleeding changes related to changes in clotting or inflammation, affecting normal menstrual repair. In this sample, 42% of people with regular menstrual cycles bled more heavily than usual, while 44% reported no change, after being vaccinated. Among people who typically do not menstruate, 71% of people on long-acting reversible contraceptives, 39% of people on gender-affirming hormones, and 66% of post-menopausal people reported breakthrough bleeding. We found increased/breakthrough bleeding was significantly associated with age, other vaccine side effects (fever, fatigue), history of pregnancy or birth, and ethnicity. Changes to menstrual bleeding are not uncommon nor dangerous, yet attention to these experiences is necessary to build trust in medicine. TeaserIncreased bleeding can occur post SARS-CoV-2 vaccines; this study characterizes patterns to enable future hypothesis testing.