According to the University of California, Santa Barbara's SexInfo Online website, it is normal for a woman to shed endometrial tissue during her menstrual period. Endometrial tissue is the inner lining of a woman's uterus.
The website explains that the endometrium grows and thickens for the duration of a menstrual cycle. Unless pregnancy occurs, hormones send signals to the endometrium, which begins the process of breaking down and shedding itself. As a result, fresh endometrial issue can grow for the next cycle. The shed endometrial tissue exits in the form of menstrual blood. SexInfo Online states that endometrial tissue can spread and grow away from the uterus, a condition called endometriosis.