On the eighth day of IVF, eight millimeters of endometrium is normal. The optimal endometrium at the time of IVF transplantation, is 8-12 millimeters. During the menstrual cycle, the endometrium will change with the ovarian hormone cycle, and the endometrium in the follicular phase will gradually thicken under the effect of estrogen. The endometrium of 10 mm on the 8th day indicates that the endometrium is better developed, and the endometrium is usually 8-12 mm at the time of implantation, which is conducive to the implantation of the fertilized egg. Before implantation, the uterine lining is relatively thin, which may easily lead to failure of implantation, or miscarriage. In this case, estrogen medication, such as estradiol valerate, needs to be given under doctor’s supervision to encourage the growth of the endometrium.