During the menstrual and proliferative phases of the menstrual cycle, the levels of estradiol and progesterone in the blood are low, thus weakening or eliminating the negative feedback effect on the pituitary and hypothalamus, leading to an increase in the secretion of gonadotropic hormones by the hypothalamus, which in turn leads to an increase in follicle-stimulating hormone and luteinizing hormone secretion by the pituitary. As a result, follicles develop and estrogen secretion gradually increases. At this time, estrogen stimulates the endometrium to enter the proliferative phase. Luteinizing hormone increases the secretion of progesterone, leading to ovulation. During this period, both estrogen and progesterone levels increase. This produces enhanced negative feedback inhibition in the hypothalamus and pituitary gland, resulting in a decrease in ovulation stimulating hormone and luteinizing hormone levels, leading to luteal degeneration and consequent decrease in estrogen and progesterone levels. The endometrium loses the support of these two hormones and flakes off and bleeds, i.e. menstruation occurs. At this point, the estrogen and progesterone decrease and the next menstrual cycle begins again. The following changes occur during the menstrual cycle: 1. When a woman enters puberty, under the control of hypothalamic gonadotropin-releasing hormone, the anterior pituitary gland secretes follicle-stimulating hormone (FSH) and a small amount of luteinizing hormone (LH) to promote follicle development and maturation in the ovaries, and begins to secrete estrogen. Under the action of estrogen, the uterine ostium undergoes proliferative changes; 2. As the follicles mature, the secretion of estrogen gradually increases, and when it reaches a certain concentration, it promotes the secretion of gonadotropins by the anterior pituitary gland through the positive feedback effect on the pituitary gland of the hypothalamus, and it is more obvious to increase the secretion of LH, forming a peak of luteinizing hormone release, which causes the mature follicles to ovulate; 3. 3. Under the action of luteinizing hormone, the post-ovulatory follicles form the corpus luteum and secrete estrogen and progesterone. During this period, the endometrium, mainly under the action of progesterone, accelerates its growth and differentiation, and transforms into the secretory endometrium; 4. As the corpus luteum secretes a large amount of estrogen and progesterone, the concentration of these two hormones in the blood increases, which inhibits the hypothalamus and pituitary gland through negative feedback, causing the pituitary gland to secrete less follicle stimulation and luteinizing hormone, and the corpus luteum then atrophies, thus progesterone and estrogen also decrease rapidly, and the endometrium suddenly loses the support of these two sex hormones. The endometrium suddenly loses the support of these two sex hormones and then collapses and bleeds, the endometrium is shed and menstruation occurs. At this time, the concentration of estrogen and progesterone in the blood decreases and the inhibition of the hypothalamus-pituitary gland is lifted, so that hypothalamic gonadotropin-releasing hormone can increase again and the next menstrual cycle begins. The first menstruation is called menarche, and the age of menarche is mostly between 13 and 15 years old. The early or late menarche is related to climate, genetics and health status. If a girl does not menstruate by the age of 18, it should be considered pathological and should be investigated and treated. Menarche marks the beginning of puberty. The function of the ovaries during puberty is still unstable, and the menstrual cycle is irregular. After the menarche, menstruation often occurs several months, half a year or even longer apart, and then gradually approaches 28-30 days. A regular menstrual cycle, the time between periods is generally not less than 20 days or more than 45 days. The normal duration of menstruation is 2 to 7 days, mostly 3 to 5 days. The average volume of blood loss is about 50 ml, but as little as 20 ml and as much as 100 ml. The menstrual blood is dark red in color because it contains fibrin, which prevents clotting of menstrual blood. Insufficient fibrin and clot formation when there is a lot of bleeding. If the menstrual period is too long or the amount of menstrual blood is too much or too little, it is abnormal and should be examined and treated. Anovulatory cycles are more common in the years after menarche and in the years before menopause, so the intervals are the longest and most variable during this period. The average amount of bleeding per cycle is 130 ml (range 13-300 ml), often the most on the second day, with a soaked menstrual pad or cotton plug absorbing 20-30 ml. Menstrual blood generally does not clot (unless there is a lot of bleeding), probably due to lyso-fibrin and other factors that inhibit clotting, based on endocrine changes. Second, the menstrual cycle can be divided into two main phases The female menstrual cycle begins with the first day of menstruation and ends with the next menstrual period. The length of the cycle varies from person to person, ranging from about 21 to 36 days, with an average of about 28 days. The duration of menstrual flow is usually 3-7 days, with an average of 5 days. The menstrual cycle is divided into a follicular phase before ovulation and a luteal phase after ovulation, separated by the day of ovulation. The length of the follicular phase varies, but the luteal phase is fixed at about 14 days on the next two days. The follicular phase extends from the first day of menstruation to the day before the peak of ovulation (LH), a period of about 12-22 days, which varies according to the body type of each person. After development and maturation, the pituitary gland starts to secrete estrogen (estrogen), which stimulates the growth of a group of 3-30 follicles, which are replenished in the last days of the previous cycle in order to accelerate growth. When the FSH level decreases, one of the replenished follicles is selected for ovulation, which matures and is then dispatched, while the rest go into decay and the first phase is over. The luteal phase is roughly the same for everyone, that is, from the day of ovulation to the next menstrual period, usually two weeks or about 14 days after ovulation is the day of menstruation. The length of the menstrual cycle depends on the length of the maturation period of the follicles in a woman’s ovaries, while the length of the luteal phase is the same, about 14 days. If the follicle development period is long, the menstrual cycle will be long and vice versa. Under normal circumstances, a dominant follicle, ≥18 mm in diameter, will be observed during the mid-menstrual period, i.e. during ovulation, when an ultrasound examination is performed. Only those with a dominant follicle, i.e. a mature follicle, will ovulate. To observe if you are ovulating, you need consecutive ultrasound tests, and one ultrasound result is not indicative of the problem. If your menstrual cycle is irregular, it will be more difficult to monitor using ultrasound. You can first go to the obstetrics and gynecology department to check your endocrine status and adjust your menstrual cycle according to the test results.