(i) Menstrual phase : Occurs on days 1-5 of the cycle. During this phase, menstruation occurs. At the beginning of this stage, level of progesterone drop dramatically because of the degeneration of the last cycle’s corpus luteum. Production of estrogen also reduces during this phase. This triggers the shedding of endometrium. The detached portion of the endometrium as well as blood passes through the vagina as the menstrual flow.

 

(ii) Follicular or proliferative phase: Proliferative phase includes days 6-13 of the cycle. Proliferative phase consists of growth of endometrium of uterus. Increased secretion of FSH causes Graafian follicles to grow, mature and secrete estrogen during this phase. The endometrium grows thicker and becomes more vascularised and glandular. Change in the levels of pituitary and ovarian hormones bring about these changes in the ovary and uterus.

 

(iii) Luteal phase : This phase includes cycle days 15-28. After ovulation and in response to luteinising hormone, the portion of the Graafian follicle that remains in the ovary enlarges and is transformed into a corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, marking a new cycle.