The thickness of the endometrium varies according to the menstrual cycle. During the menstrual period, the endometrium is at its thinnest state, when its thickness is around 2mm as detected by ultrasound. During the proliferative phase of the endometrium, with the continuous stimulation of estrogen, the thickness of the endometrium grows, and during ovulation the endometrial proliferation is at its fullest state. After ovulation, due to the action of progesterone, the endometrium during the proliferative period will show a secretory response, on top of which the thickness of the endometrium will grow further. During the 7-8 days after ovulation, the thickness of the endometrium will increase further. The thickness will be around 10mm by ultrasound. Later, before menstruation, the endometrium thins as the estrogen as well as progesterone levels fall. This is the normal evolution of the endometrium. In abnormal cases, due to single estrogen stimulation, the endometrium is in a state of abnormal hyperplasia, or even atypical hyperplasia or malignant hyperplasia, which should be treated promptly.