On the 14th day of the menstrual cycle, the thickness of the endometrium is about 5mm, which is the proliferative phase of the endometrium. By the 15th-28th day of the menstrual cycle, it is the secretion phase of the endometrium, when the endometrium is thicker and fluffier, containing very rich nutrients, which is due to the fact that the body’s secretion of estrogen and progesterone is particularly high at this time, and the glands of the endometrium are also more curved, presenting the phenomenon of secretion, and all of these changes are conducive to the fertilization of the egg in the uterine cavity to bed. If a woman’s uterine lining is too thin, the uterus will not have enough nutrients, and the fertilized egg will not be able to implant in such a uterine cavity, which will easily lead to miscarriage.