The endometrium is normally 0.3-0.5cm when the menstrual period has just been cleared. At this time, with the development of the follicle, the level of estrogen in the body will rise, and the endometrium will gradually thicken under the action of estrogen. By the time ovulation occurs, the thickness of the uterine lining can reach more than 8mm, between 8-15mm is normal, this time the endometrium is too thin or too thick on the pregnancy will have an impact. If the endometrium is too thin, the gestational sac may not be able to implant, and thus pregnancy may not be possible; if the endometrium is too thick, the gestational sac will be farther away from the basal layer of the endometrium after implantation because the blood supply of the endometrium is in the basal layer of the endometrium, and if the blood vessels are farther away from the basal layer, they will not be able to supply to the gestational sac, and thus miscarriage may also occur.