Why does Menstruation occur?

Answer: Menstruation is a common occurrence in women characterized by normal vaginal bleeding and is part of the monthly cycle of a healthy woman. A healthy woman experiences this 28-day cycle until she reaches menopause. 

It happens every month during menstruation as it prepares the body for pregnancy. Once the egg is discharged from the ovaries, the uterine lining thickens so that the embryo may implant.