Decidual cast shedding (DCS) -- clinically known as membranous dysmenorrhea -- is a rare gynecological event during which the decidua, or the thick mucous membrane that lines the uterus, is shed in full, intact form. The "cast" that is shed from the vagina is fleshy and red or pink in color, typically with a triangular shape that mirrors the internal shape of the uterine cavity.
For over the preceeding 100 years, fewer than 40 cases of decidual cast shedding were reported. However, over a 7.5-month period in 2021, 292 women experienced it (MyCycleStory survey distributed via social media from May 16, 2021, to December 31, 2021), raising questions about whether COVID-19 vaccines could be to blame.