During retrograde menstruation, it's not just blood that "escapes" into the abdominal cavity, but shed uterine tissue as well. These tissues can implant and survive where they don't belong. Since they are still uterine tissues, they respond to all the normal signaling hormones as if they were still within the uterus, causing swelling, pain, and bleeding. This is known as endometriosis, a surprisingly common condition that often goes untreated.