The endometrium, which surrounds the uterine lining, is a mucous membrane located in the uterine cavity that changes in thickness from puberty onwards with the cycle of estrogen and progesterone secreted by the ovaries, and is not constant. On average, the process of endometrial peeling and bleeding followed by repair and proliferation occurs every 28 days and is called the menstrual cycle. The endometrial cycle is divided into 3 phases: the menstrual phase, the proliferative phase and the secretory phase. The endometrium has 3 layers: the dense layer, the spongy layer and the basal layer. The dense layer and the spongy layer, collectively known as the functional layer, are the 2/3 of the surface of the endometrium and are subject to hormonal stress and cyclical changes, while the basal layer, the 1/3 of the endometrium near the myometrium, is fixed and is not subject to hormonal influence and does not normally undergo cyclical changes. The endometrium is one of the most differentiated tissues in the body and its main role is to form menstruation in the absence of pregnancy and to proliferate rapidly during pregnancy to provide nutrition for embryonic growth and development. The endometrium undergoes the process of shedding, growing, becoming fertile and then shedding during the three different phases: menstruation, proliferation and secretion. The thickness of the endometrium changes dynamically throughout the menstrual period from about 0.5mm just after menstruation to about 14mm just before menstruation again. The following is an example of a menstrual cycle of 28 days: 1. The wonderful cyclical changes of the endometrium 1. Due to the degeneration of the corpus luteum in the ovary, the secretion of estrogen and progesterone suddenly decreases, and the spiral arteries in the functional layer of the endometrium undergo continuous contraction, resulting in endometrial ischemia and tissue necrosis. After the contraction, the spiral artery suddenly and briefly expands, and the blood overflows into the connective tissue, eventually breaking through the surface layer of the degenerated and necrotic endometrium, flowing into the uterine cavity and being discharged from the vagina, which is the menstrual blood. At this time, the endometrium is in a state of shedding, it is very thin, about 1-4mm, and looks like a thin line, not too clearly demarcated from the myometrium. At this time, a group of fresh basal follicles are also gradually awakening, ready for the “baby-making” project. The proliferation phase, also known as the follicular phase, lasts for 5-14 days of the cycle. As the menstrual cycle ebbs, several follicles grow in the ovaries during this period. Under the action of estrogen secreted by the follicles, the endometrium undergoes proliferative changes and gradually thickens, and the endometrium is repaired before the termination of menstruation. The epithelial cells and stromal cells continue to divide and proliferate throughout the proliferative period, and the uterine glandular cells are more responsive to hormones, with estrogen causing the gradual growth and differentiation of the glandular epithelium. By the late proliferative stage (11-14 days), the size may increase to about 9-14 mm at one time. At this point, the fresh follicles undergo a screening process and eventually one follicle stands out and grows larger and larger towards maturity, while the other follicles undergo atresia and exit the stage of history. The secretory phase, also known as the luteal phase, is the 15-28 days of the menstrual cycle. At this time, the ovaries have ovulated and the corpus luteum is formed. The endometrium continues to thicken under the effect of estrogen and progesterone secreted by the corpus luteum, especially progesterone, and becomes more and more lax, like a soft sponge warming bed, releasing nutrients, so that if the egg and sperm unite, the embryo formed will find the warmest corner in this warm bed and quietly settle down. If the mother-to-be does not have a helper this month, the endometrium will shed again after the 28th day of the menstrual cycle to usher in a new era. II. Normal female endometrial thickness The endometrial thickness is closely related to reproduction and endocrine. Under normal pregnancy, the endometrium is the place where the fertilized egg is laid, and too thin or too thick is not conducive to fertilization. However, the thickness of the endometrium is to some extent relative and there is no definite criterion to say to what extent the endometrium is thin enough to cause infertility or to what extent it is thick enough to cause infertility. For women of reproductive age with a menstrual cycle of 28-30 days, the reference standard for endometrial thickness is 4mm-5mm in the follicular phase, while for ovulation, the reference standard is 8mm-10mm, and for the luteal phase, the reference standard is 10mm-12mm. At different times, the endometrial thickness and mint pattern are different. The evaluation of endometrial thickness should be based on a comprehensive assessment of the patient’s past medical history, endocrine status, warning cycle, medication and other factors. The common clinical causes of endometrial thinning are due to pathogenic infection, mechanical injury, mental and psychological factors, and endocrine dysfunction; endometrial thickening may be a pathological condition and should be combined with hysteroscopy for diagnostic scraping to do pathological examination on it. Third, the endometrium is too thin and too thick harm everything in moderation. This is also true for the endometrium, which can be too thin or too thick, leading to bad effects. If the embryo is compared to a “seed”, then the endometrium is the “soil”, and a good “seed” needs a suitable “soil” to develop and grow. “If the “soil” is not enough nutrients, the “seed” will die if it has no nutrients to absorb, and if the “soil If the “soil” is not enough nutrients, the “seed” will have no nutrients to absorb and will die, and if the “soil” is too sufficient, it will lead to too much nutrients for the “seed”, which is also not conducive to the survival of the “seed”. If the endometrium is too thin, the fertilized egg will not be able to settle well, and even if it does, it will easily lead to miscarriage or stillbirth. If the endometrium is too thick, it may be a pathological phenomenon, such as anovulatory uterine bleeding, submucosal fibroids, endometritis, endometrial polyps, etc. These diseases may also lead to the risk of infertility.