The thickness of the endometrium is usually around 8-12mm which is easily suitable for fertilization of the egg, the average being around 10mm. The thickness of the endometrium varies throughout the menstrual cycle as hormone levels change during the menstrual cycle. The endometrium can be divided into menstrual phase, proliferative phase, and secretory phase. 1. Menstrual phase: During the first 1-4 days of menstruation, the corpus luteum degenerates and estrogen and progesterone levels decrease, resulting in the shedding of the endometrium, when the thickness of the endometrium drops to about 2mm. 2. Proliferative phase: It usually starts on the 5th day of menstruation and grows to about the 14th day. The proliferative phase is when the endometrium slowly grows thicker under the action of estrogen, and may grow to about 3-5mm; 3. The secretory phase: around day 14-28 of the menstrual cycle, which is also the luteal phase, basically after ovulation, when progesterone is at a high level. At this time, the endometrium grows to a more appropriate thickness under the action of estrogen, usually 8-12mm, with an average of about 10mm. At this time, the endometrium is fluffy and rich in nutrients, which is suitable for the fertilized egg to be laid. However, in some people, because of low hormone levels, especially low estrogen levels, the endometrium does not grow to the right thickness and may be prone to miscarriage or non-pregnancy after pregnancy. In some people, the endometrium can be very thin because the basal and functional layers of the endometrium have been damaged, and the use of estrogen is not effective at this time. Therefore, it is best for those who do not have a need to have children to use good contraception and avoid abortions so that they do not conceive easily when they want to get pregnant because of problems with the endometrium. When the thickness of the endometrium exceeds 15mm, it is considered as endometrial thickening, which can affect pregnancy. A pathological examination should be done in time to determine the cause of endometrial thickening and exclude the possibility of malignant tumor.