The endometrium is the soil in which the embryo is implanted and undergoes cyclic growth and shedding with the menstrual cycle. During follicle monitoring under ultrasound, we measure the thickness of the endometrium and evaluate the morphology each time. During menstruation, the endometrium is linear, a sign of normal endometrial exfoliation. As follicle development estrogen levels increase, the endometrium gradually thickens. The endometrial thickness we are concerned with is the thickness of the endometrium measured close to ovulation, or on the HCG day of the ovulation promotion cycle, which is the day of an injection of human chorionic gonadotropin. Endometrial thickness and morphology are equally important indicators. Generally, an endometrial thickness greater than 8 mm and a morphology of A – clear trilinear shape are the most suitable for embryo implantation. There are also many studies that consider greater than 7mm as an indicator of moderate endometrial thickness. The definition of thin endometrium varies from less than 6mm to 9mm, and this inconsistency is related to individual differences in different populations as well as to the measurement methods and errors. After ovulation or HCG injection, the endometrium transforms from the proliferative phase to the secretory phase, and we no longer measure the endometrium. It has been shown that in a natural cycle, most women younger than 40 years old can have an endometrium of 9mm or more, and only 5% have an endometrium <9mm; while 25% of women between 41 and 45 years old have an endometrium <9mm. The highest pregnancy rates are obtained with an endometrium thickness between 9 and 13mm. One of the possible reasons for the relative decrease in pregnancy rates and increase in biochemical pregnancy rates for endometrium <9mm or >13mm is unclear: thicker endometrium is more susceptible to mechanical damage during transplantation. The traditional view is that an endometrium of <6 mm is not suitable for embryo transfer, while recent studies have continued to refresh the lower limit of endometrial thickness, with numerous reports of successful pregnancies in patients with an endometrial thickness of 4-5 mm, so it is argued that ≤4 mm should be used as a criterion for cancellation of transfer. However, there is currently a great deal of controversy as to what endometrial thickness is acceptable for embryo transfer without loss of pregnancy rate. Although there are precedents of successful pregnancies with very thin endometrium in our clinic, too thin endometrium will definitely affect the chances of pregnancy relatively. Therefore, in clinical practice, in order not to waste valuable embryos, transfer is not recommended for patients with an endometrium of <6 mm, unless multiple treatments have failed. The endometrial factor is one of the most important factors affecting pregnancy, and each person's individuality dictates that the same objective indicator cannot be used to determine what endometrial thickness is most likely to result in pregnancy. Therefore, the doctor will guide the patient based on the patient's past medical history and the specific circumstances of the thickness and morphology of the lining.