The menstrual cycle is a monthly cycle of changes in the ovaries and the lining of the uterus (endometrium), starting with the preparation of an egg for fertilization. The whole cycle takes around 28 days to complete. When the egg is not implanted in the uterus the endometrium of the uterus is shed and is released in the form of blood and mucous through the vagina. This is known as menses.
The hormones that regulate the menstrual cycle are follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone. During the follicular phase, the level of LH and FSH increases under the influence of RH (releasing hormone) from the hypothalamus. These stimulate the conversion of a primary follicle into a Graafian follicle. The level of LH increases gradually leading to the growth of follicle and secretion of estrogen. Estrogen inhibits the secretion of FSH and stimulates the secretion of luteinizing hormone. It also causes the thickening of the uterine endometrium. The increased level of LH causes the rupturing of the graafian follicle and release the ovum into the fallopian tube. The ruptured Graafian follicle transform into corpus luteum and starts secreting progesterone hormone during the luteal phase. Progesterone hormone helps in the maintenance and preparation of endometrium for the implantation of the embryo. High levels of progesterone hormone in the blood decrease the secretion of LH and FSH, therefore inhibiting further ovulation.