Menstruation is the regular shedding of epithelial cells from the uterus. It happens in all female mammals of child bearing age. In human females, it occurs approximately once every 28 days from the onset of puberty to the onset of menopause.
The cells of the uterus become prepared for the implantation of a fertilized egg in conjunction with a female's fertilization cycle. However, if the egg is not fertilized, the cell wall must be shed in order to prepare for the next egg. If a female becomes pregnant, she does not menstruate and this is a sign that a woman may be pregnant. However, there are other disorders that can interfere with the menstrual cycle.
Women taking the pill continue to have a menstrual cycle, but do not release eggs. Generally, women on the pill have less shedding of cells than woman who are not.
See also endometriosis, a condition where uterine cells migrate to other parts of the body.