Menstruation phase occurs when the concentration of ovarian and gonadotropin hormones is low in blood. Reduced concentration of these hormones stimulates the hypothalamus to produce GnRH (gonadotropin-releasing hormone). 

It activates anterior pituitary to gonadotropin hormones.

(i) Pituitary hormones : The pituitary hormones secreted during menstrual cycle are FSH and LH. The FSH (Follicular stimulating hormone) stimulates the growth of primary oocyte into secondary oocyte and the Graafian follicle.

The maximum level of LH (Luteinising hormone) induces rupture of Graafian follicle to release ovum containing secondary oocyte (ovulation).

(ii) Ovarian hormones : The FSH secretion by pituitary gland stimulates the ovarian follicles to secrete oestrogen. Oestrogen stimulates the proliferation of endometrium of the uterine wall. After ovulation, the remaining cells of ovarian follicle are stimulated by LH to develop corpus luteum which secretes large amount of progesterone. Progesterone is essential for maintenance of endometrium, for implantation of fertilised ovum and other events of pregnancy.