What happens to the uterine lining between day 27 and day 1 and what is this process called?

Between day 27 and day 1 of the menstrual cycle, if pregnancy does not occur, the uterine lining (endometrium) begins to break down and is shed. This process, called menstruation, involves hormonal changes that lead to the constriction of blood vessels in the lining, causing it to disintegrate and be expelled from the body. This shedding typically lasts for about 3 to 7 days, marking the start of a new menstrual cycle.