Once a month, just after a woman releases the egg (at ovulation) from her ovary, the lining (endometrium) of the womb (uterus) is at its peak to allow the embedding of a fertilised egg.
If pregnancy does not occur, the endometrium starts to deteriorate as the hormones that sustain it in peak condition alter. After a few days, the lining breaks down completely, sloughs off the wall of the uterus, and is washed away by the blood released from the arteries that supplied it in a process known as menstruation or the menses. Contractions of the uterus help remove the debris.
After three to five days, the bleeding stops, and a new lining starts to develop ready for the next month’s ovulation.