This is because the thickness of the endometrium is not static and changes cyclically with the secretion of ovarian hormones. On the 15th day of menstruation, the thickness of the endometrium is about 1.0-1.2cm. At the time just after menstruation, the thickness of the endometrium is about 0.6-0.7cm. As the menstrual period progresses to ovulation, the endometrium gradually thickens, so that on the 15th day of menstruation, which coincides with ovulation, the endometrium reaches a certain thickness before the fertilized egg can be deposited. During the secretory phase, the endometrium gradually thickens, for example, by the premenstrual period, the endometrium is about 1.2cm to 1.4cm.