Uncovering the mystery of women's menstrual cycle

During each menstrual cycle, the ovaries undergo a series of morphological and functional changes, including the development, maturation, ovulation, corpus luteum formation, and cyclical secretion changes of estrogen and progesterone.

Changes in the secretion of these two sex hormones cause the endometrium to undergo a series of morphological and functional changes. These changes in the ovaries are caused by changes in the secretion of gonadotropins by the pituitary gland, which in turn is controlled by lower brain hormones. In addition, estrogen and progesterone secreted by the ovaries can also exert feedback effects on the pituitary gland and hypothalamus to regulate their activities.

Changes in the endometrium during each menstrual cycle can be divided into 3 stages.

(1) Menstrual period lasts 4-5 days

As the levels of progesterone and estrogen in the blood drop to the lowest level, the prostaglandins produced by the uterus together cause the spiral arteries to spasm, thereby disrupting the blood supply to the endometrium. This will cause the endometrium and its blood vessels to necrotize and fall off, and the resulting bleeding can flush the dead endometrium into the uterine cavity. On the other hand, the increase in prostaglandins can also lead to contraction of the myometrium, which helps to discharge blood and endometrium from the uterine cavity, but can also cause dysmenorrhea that accompanies menstruation.

(ii) The proliferative phase (also called the follicular phase in terms of ovarian changes) lasts about 10 days, which is around the 5th to 14th day of the menstrual cycle.

During this period, as the follicles grow, they secrete more and more estrogen, which gradually increases the estrogen level in the blood, thereby repairing the endometrium. As a result, the endometrium gradually grows and thickens, and the uterine glands also grow.

(III) The secretory phase is also called the luteal phase in terms of ovarian changes. It lasts about 14 days, that is, the 15th to 28th day of the menstrual cycle.

This period is the result of the progesterone secreted by the mature follicle that forms a corpus luteum after ovulation at the end of the previous period, which acts on the endometrium. Progesterone can cause blood vessel proliferation and congestion in the thickened endometrium, and cause the uterine glands to expand and bend to secrete mucus. These changes in the endometrium facilitate implantation (conception). If pregnancy does not occur during this menstrual cycle, the corpus luteum will atrophy due to lack of support from the relevant hormones, and will stop secreting progesterone and estrogen. When the levels of these two sex hormones in the blood drop to extremely low levels, the thickened endometrium undergoes necrosis, bleeding and shedding due to the loss of hormonal support, and the menstrual period begins.

Although the series of changes in the menstrual cycle occur automatically in the body, human psychological factors can affect this automatic cyclical change through the lower brain. For example, drastic mood swings, environmental changes, or suspicion of pregnancy can all cause menstrual cycle disorders.