Ovarian function during a normal menstrual cycle passes through a follicular-preovulatory phase and a letulal-postovulatory phase. The characteristics of the endometrium also change and go through a proliferative-preovulatory and secretory-postovulatory phase. The regular onset of menstruation requires precise temporal and quantitative regulation of the balance of multiple sex hormones. The axis hypothalamus-pituitary-ovaries is of leading importance. The hypothalamus provides a pulsatile release of gonadotropin-releasing hormone. It stimulates the synthesis and secretion of the letulinizing – LH and follicle-stimulating – FSH hormones from the anterior lobe of the pituitary gland. These hormones act on ovarian function and regulate the development of follicles and the release of hormones from the ovaries, namely estrogen, progesterone and androgens. The development of a mature follicle creates a rapid rise in estrogen levels and leads to an increase in LH concentration. These conditions are necessary for the normal course of ovulation. After ovulation, this hormone stimulates luteinization and allows the formation of the corpus luteum. It releases estrogen but also high levels of progesterone. At conception, the function of the corpus luteum is supplemented by human chorionic gonadotropin secreted by the early placental profoblast. It is a structural analogue of LH and favors the maintenance function of the corpus luteum for pregnancy. If conception does not occur, there is a regression in the development of the corpus luteum and a decrease in the levels of progesterone and estrogen. The absence of menstruation – amenorrhea is due to dysregulation in the described mechanisms, but also to some other acquired conditions. Cervical stenosis may be due to postoperative scar formation and change in its architectonics after dilatation, curettage, conization, infections, or neoplastic diseases. Stenosis affects the inner surface of the cervix and can cause a lack of menstruation, abnormal bleeding, dysmenorrhea or even infertility. Intrauterine adhesions are due to various processes that lead to the appearance of scars, dense adhesions and complete obliteration of the uterine cavity. The endometrium consists of two layers – functional, which covers the endometrial cavity, and basal, which ensures the regeneration of the functional layer during each menstrual cycle. Regular destruction of the basal endometrium prevents its thickening in response to ovarian hormones. Damage to the structure of the endometrium can be due to interventions such as curettage, miscarriage or abortion followed by infection. In some cases, this can be observed with a cesarean delivery or infection with the use of intrauterine contraceptives. Depending on the degree of scar formation and adhesions, patients may experience amenorrhea, hypomenorrhea, or recurrent miscarriages due to disturbances in the processes of placenta formation. Another reason for lack of menstruation is hypergonadotropic hypogonadism.This condition is characterized by reduced or absent ovarian function and elevated serum concentrations of LH and FSH. The condition is due to ovarian dysfunction and there are no abnormalities in the functions of the hypothalamic-pituitary axis. This process is sometimes referred to as premature menopause or premature depletion of the ovarian reserve. In these conditions, a change in the functions of the ovaries is observed before the reserve is finally exhausted, which is manifested by disturbances in the regularity of the menstrual cycle. Hypergonadotropic hypogonadism can be an acquired condition resulting from infection, exposure to harmful environmental agents, or autoimmune processes. Some of the leading factors from everyday life are smoking, exposure to chemicals, heavy metals, pesticides and solvents. Ovarian dysfunction may result from autoimmune involvement of the pituitary gland, which is often associated with hypothyroidism and adrenal insufficiency, or may result from systemic lupus erythematosus. In some cases, ovarian dysfunction can be observed after surgical interventions due to removal of the ovaries and ovarian cysts, endometriosis or severe forms of pelvic inflammatory disease. Absence of menses may also be observed in patients treated with chemotherapy and radiotherapy for neoplastic or autoimmune diseases. References: Williams Gynecology, 4th Edition, McGraw Hill Education, 2020