The ovaries besides producing eggs also secrete two hormones:
(a) Progesterone, and
(b) Estrogen.
These hormones help in the secretion of corpusluteum. If the egg is not fertilised, then corpusluteum of ovary degenerates. Periodically, the quantity of progesterone hormone decreases in the blood, resulting in the bursting of blood vessels and the blood along with uterine fluid comes out. This is called menstruation. This process goes on in a cyclic manner, after every 28 days. It is called the menstrual cycle.