(i) Menstrual phase: The cycle starts with this phase and lasts for 3-5 days. It results due to the breakdown o{ endometrial lining of the uterus and its blood vessels. 

(ii) Follicular phase: Proliferative phase (primary oocyte transforms into Graafian follicle) and endometrium regenerates through proliferation, level of LH and FSH increases and stimulates secretion of estrogen by growing follicles, maximum leve1 of LH induces rupture of Graafian follicle to release ovum containing secondary oocyte (ovulation). 

(iii) Luteal phase : Remnants of Graafian follicle forms corpus luteum, which secretes large amount of progesterone, essential for maintenance of endometrium, a necessity for implantation and for pregnancy. During pregnancy, all events of menstruation cycle stop and no menstruation occurs. In the absence of fertilisation, corpus luteum degenerates leading to disintegration of endometrium and resulting in menstruation flow. There is low level of progesterone. Menstrual cycle ceases at 50 years of age and termed as menopause.