Why does menstruation occur?

Every month, a woman's body prepares the uterus for a possible pregnancy by building up a thick lining. If the egg isn't fertilised, this lining isn't needed, so the body sheds it. This shedding is menstruation.

Menstruation is a monthly process in females of reproductive age. It occurs as follows: 1. Each month, an ovary releases an egg (ovulation), and the uterus prepares to receive a fertilised egg by thickening its inner lining (endometrium) with blood vessels and nutrients. 2. If the egg is fertilised, it implants in the uterine lining and pregnancy begins. 3. If fertilisation does not occur, the egg disintegrates, and the thick uterine lining is no longer needed. 4. The levels of progesterone drop, causing the blood vessels in the endometrium to constrict. 5. The lining breaks down and is shed along with blood, mucus, and the unfertilised egg out through the vagina. This cycle repeats approximately every 28 days from puberty until menopause (around 45-50 years of age).