Menstruation is a natural process of the uterus shedding its lining when no pregnancy occurs. During the menstrual cycle, the endometrium (inner lining of the uterus) thickens to become an optimal environment for the implantation and growth of an embryo. If no pregnancy occurs, the endometrium would be shed and discharged through the vagina. About half of the menstrual blood is the blood produced by the tearing of the blood vessels when the endometrium is shed, and the other half is the tissue of the endometrium, cervical mucus and vaginal secretions.
The menstrual cycle is 21 to 35-day long (on average 28-day), while bleeding usually lasts for 3-5 days (2-7 days is still considered normal). Apart from ovulation which happens on days 12 to 16 in a 28-day cycle (counting the first day of menstruation as day 1) and menstruation, the menstrual cycle also brings us various hormonal changes. These hormonal changes can be accounted for mood swings, headache, insomnia and edema etc., which are also referred to as PMS (premenstrual syndrome).