Oestrogen concentration remains almost constant and produce throughout the menstrual cycle Generally, menstrual cycle have four phases $(i)$ Menstrual phase $(a)$ The soft tissue of endometrial lining of the uterus disintegrates causing bleeding.$(b)$ The unfertilized egg and soft tissue are discharged. $(c)$ It lasts $3-5\; days$. $(ii)$ Follicular Phase/Proliferative Phase $(a)$ The primary follicles in the ovary grow and become a fully mature Graafian follicle.$(b)$ The endometrium of the uterus is regenerated due to the secretion of $\mathrm{LH}$ and FSH from anterior pituitary and ovarian hormone, estrogen.$(c)$ It least for about $10-14$ $days$.$(iii)$ Ovulatory Phase $(a)$ Rapid secretion of $LH$ (LH surge) induces rupture of Graafian follicle, thereby leading to ovulation (release of ovum).$(b)$ It lasts for only about $48\; \mathrm{hr}$.$(iv)$ Luteal Phase/Secretor Phase $(a)$ In this phase the ruptured follicle changes into corpus luteum in the ovary and it begins to secrete the hormone progesterone.$(b)$ The endometrium thickens further and their glands secrete a fluid into the uterus.$(c)$ If ovum is not fertilized, the corpus luteum undergoes degeneration and this causes disintegration of the endometrium leading to menstruation.$(d)$ Oestrogen and progesterone levels rise during this phase. It lasts for only $1$ $day$. $(e)$ During pregnancy all events of the menstrual cycle stop and there is no menstruation. The menstrual cycle permanently stops in females at the age of around $50$ $years$. This is called menopause