→ (i) Menstrual phase:
→ The cycle starts with this phase, when menstrual flow occurs and it lasts for 3-5 days.
→ The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina.
→ Menstruation only occurs if the released ovum is not fertilised
→ Lack of menstruation may be indicative of pregnancy.
→ However, it may also be caused due to some other underlying causes like stress, poor health etc.
→ (ii) Follicular phase / Proliferative phase: (Day 3-5)
→ The menstrual phase is followed by the follicular phase.
→ During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation.
→ These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.
→ The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles.
→ (iii) Ovulatory phase: (Day 6-14)
→ Both LH and FSH attain a peak level in the middle of cycle (about 14th day).
→ Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation).
→ (iv) Luteal phase / Secretory phase: (Day 15-28)
→ The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum.
→ The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy.
→ During pregnancy all events of the menstrual cycle stop and there is no menstruation.