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The cyclic events starting from one menstruation till the next that take place during the reproductive period (from puberty to menopause) of a woman's life is called menstrual cycle. The whole cycle takes around 28 or 29 days to complete.
Menstrual cycle has the following phases :
1. Menstrual phase : The cycle starts with menstrual flow (bleeding). It lasts for 3-5 days. It is due to breakdown of endometrial lining and blood vessels of the uterus that comes out through vagina. Menstruation occurs if the released ovum is not fertilized.
(ii) Follicular (Proliferative) phase : In this phase, the primary follicle matures into Graafian follicles under the influence of FSH and LH. The developing follicle releases estrogen which causes the thickening of the uterine endometrium.
(iii) Ovulatory phase : At the time of ovulation, the release of gonadotropins (LH and FSH) increases. The LH and FSH are at their peak in the middle of the cycle (14th day). The increased level of LH causes the rupturing of Graafian follicle and release of ovum into the fallopian tube (ovulation).
(iv) Secretory (Luteal) phase : After ovulation, the remains of the Graafian follicle get converted into the yellow endocrine mass called Corpus luteum. It secretes progesterone. Progesterone hormone helps in maintenance and preparation of endometrium for the implantation of the embryo. During pregnancy all events of menstrual cycle stop and there is no menstruation. In the absence of fertilization, corpus luteum degenerates and becomes a whitish mass called corpus albicans. It leads to menstruation indicating a new cycle.
(i) Menstrual Phase : Menstrual flow occurs / due to breakdown of endometrial lining of uterus , when fertilization does not occur.
(ii) Follicular Phase : Primary follicles grow into mature graafian follicles and endometrium regenerates through proliferation, changes induced by pituitary and ovarian hormones.
(iii) Ovulatory Phase : LH surge , induces rupture of graafian follicle and release of secondary oocyte /ovum during middle of cycle (i.e. 14th day).
(iv) Luteal phase : Ruptured graafian follicle transforms into corpus luteum which secrete large amount of progesteron, essential for maintaining endometrium.