Endometrial thickness is the thickness between the endometrium and the endometrial muscle layer. Under ultrasound, endometrial thickness during menstruation: 1-4mm, mid-proliferative phase: 4-8mm, late follicular phase: 8-14mm, and luteal phase is 7-14mm . There is no unified consensus on the definition of thin endometrium. The oral contraceptive pill maintains a thin and smooth endometrial environment, even in the late follicular phase where the endometrium is only 1-3 mm thick; on the contrary, if the endometrium is less than 6 mm thick, pregnancy is practically impossible from the point of view of infertility. An endometrial lining of at least 7-8 mm is a prerequisite for embryo implantation. In IVF-ET cycles, increasing endometrial thickness is associated with increased pregnancy rates. Data from donor programs show that pregnancy is unlikely to occur if the endometrial thickness is less than a specific lower limit. Although there are some reports of successful embryo transfer with an endometrial thickness of 4 mm, most cases suggest that an endometrial thickness of at least 6 mm is a prerequisite for a successful transfer, and some reports suggest that an endometrial thickness of 7 mm or even 8 mm is necessary for a successful transfer.