What Is the Most Common Menstrual Cycle Length?

After a woman enters puberty, the endometrium is affected by ovarian hormones, and periodic uterine bleeding occurs, which is called menstruation. The cyclical changes in the endometrium are called the menstrual cycle and also the human reproductive cycle.

Menstrual cycle

After women enter puberty,

Usually the first day of menstruation to the day before the next menstruation is called a menstrual cycle, each menstrual cycle averages about 28 days. According to the histological changes of the endometrium, the menstrual cycle is divided into three stages: (1) the proliferative phase (pre-ovulation phase, follicular phase). Corresponds to the 5th to 14th day of the menstrual cycle. Follicles secrete estrogen, which gradually repairs and thickens the endometrium, blood vessels and glands proliferate, and follicles develop until mature ovulation. (2) The secretory period. From ovulation to before the next menstruation, the 15th to 28th day of the menstrual cycle. The corpus luteum grows mature and secretes a large amount of progestin and estrogen. Under the action of hormones, the endometrium and glands continue to grow and secrete mucus, preparing conditions for the fertilization and development of fertilized eggs. (3) Menstrual period. If the egg cells are not fertilized, the corpus luteum gradually shrinks, hormone secretion decreases sharply, endometrial vasospasm, and the endometrium ischemia and necrosis and strip. Vascular rupture and bleeding, blood and exfoliated intimal fragments are excreted through the vagina. It lasted about 3 to 5 days and the bleeding volume ranged from 50 to 100 ml. Generally, a woman begins to have her first menstruation at the age of 13 to 14, which is called menarche. 45 to 50 years is the termination period. The human nervous system has a regulating effect on ovarian activity and can cause disturbances in the menstrual cycle during stress or excessive anxiety. There is usually no menstruation during pregnancy and lactation.

The interval between the first days of two periods. It is usually within 28 to 30 days, and it is a normal range about 3 days in advance or postponed. The length of the cycle varies from person to person, but each woman's menstrual cycle must have her own regularity, otherwise it should be considered abnormal. The menstrual cycle is regulated by the interaction between the hypothalamus-pituitary-ovary. The hypothalamus regulates pituitary function, which in turn regulates ovarian function. The endometrium undergoes periodic changes under the action of ovarian hormones. The sex hormones produced by the ovaries in turn affect the hypothalamus and pituitary gland, affecting the release of gonadotropin-releasing hormone (Gn-RH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), that is, feedback. When it is released, it is called negative feedback, and when it is released, it is called positive feedback.

After the luteal atrophy of the first menstrual cycle, the secretion of estrogen and progesterone decreased, which relieved the inhibition of the hypothalamus and pituitary. Gn-RH produced by the hypothalamus promotes the secretion and release of FSH and LH. With the synergistic effect of FSH and LH, the follicles in the ovaries gradually mature and produce estrogen, which changes the endometrial hyperplasia. After the follicles mature, the first peak of estrogen in the body appears. Increased estrogen secretion, inhibited FSH production, promoted increased LH secretion, LH peaks appeared, and triggered ovulation. After ovulation, the corpus luteum is formed, and estrogen and progesterone are secreted. Under the combined action, the uterine endometrium undergoes typical secretory changes. After ovulation, the estrogen level temporarily decreased, and then a second lower peak appeared. The large amount of estrogen and progesterone secreted by the corpus luteum reduces the FSH and LH secretion through negative feedback, and the corpus luteum begins to shrink. Thereafter, the secretion of estrogen and progesterone decreased, and the endometrium could not receive the support of sex hormones. Necrosis and shedding occurred and menstrual cramps occurred. After the corpus luteum atrophy, the inhibition of the hypothalamus and pituitary is released, and Gn-RH is secreted again to start another menstrual cycle.

1. The main cause of dysfunctional uterine bleeding is dysregulation of hypothalamic-pituitary-ovary axis regulation. For adolescent dysfunction, due to the immature regulatory function of the hypothalamus pituitary and stable cyclic regulation and positive feedback with the ovaries, FSH continues to be low, LH has no peak formation, and follicles have grown but not ovulated. Follicular atresia. Low estradiol (E2) does not form a positive feedback. During menopause, ovarian function declines. The number of follicles decreases significantly. Follicles are less sensitive to gonadotropins, E2 secretion is significantly reduced, and FSH levels are increased. Because they cannot reach the positive feedback level of estrogen, there is no pre-ovulation LH peak, and anovulatory dysfunction occurs. There are multiple mechanisms for dysfunction in childbearing age. For example, due to the influence of physical conditions, the feedback mechanism does not coexist; the conversion of peripheral androgens into estrone increases the level of E2 converted from estrone, and destroys the periodic changes of E2. Distortion of FSH / LH ratio, no LH peak, no ovulation, etc. There are also unknown reasons for the low FSH / LH ratio, which affects follicle maturation, E _{2 is} too low without causing positive feedback, and there is no ovulation. The principle of treatment is to cyclically or sequentially supplement estrogen and progestin to achieve hemostasis or periodic menstruation. Women of childbearing age can also promote ovulation and establish a normal follicular development cycle.

2. Polycystic ovary syndrome (PCOS) is one of the most common gynecological endocrine diseases. Recent research estimates that the incidence of PCOS in women of childbearing age is about 4% to 12%. The main clinical features of PCOS are ovulation disorders, irregular menstruation or amenorrhea, infertility, hairy, acne, obesity, etc. Its typical endocrine characteristics are: high LH / FSH ratio, hyperandrogenemia, hyperestroneemia, hyperprolactinemia, insulin resistance, etc. The main treatment direction of PCOS is to reduce LH and androgen levels, improve insulin resistance, and promote ovulation in small doses to establish the normal microenvironment of follicle development.

The application of the menstrual cycle regulation mechanism is of great significance to the improvement of ovulation promotion programs. Estrogen and progestin combined oral contraceptives starting from the follicular phase or estrogen supplementation starting from the late luteal phase can suppress the increase of FSH and GnRH through negative feedback, avoid premature selection of dominant follicles and improve the synchronization of follicular development in the ovulation-promoting cycle. In patients with decreased ovarian reserve, the choice of dominant follicles may appear earlier. Under the stimulation of high-dose FSH in the ovulation-promoting cycle, the dominant follicles rapidly increase, and then they are blocked due to the difference in diameter from other follicles. In this type of patients, early application of the hormone replacement cycle to inhibit FSH elevation can not only increase follicle synchronization, but also increase the reactivity of granule cells to exogenous FSH due to estrogen supplementation, thereby increasing the number of mature eggs obtained. The first line of ovulation-promoting drug clomiphene is to increase the endogenous FSH secretion by binding to the pituitary estradiol receptor, inhibiting the negative feedback of E2 on FSH, and promoting the development of follicles.