Endometrial thickness of 4mm means that the thickness of the endometrium is 4mm. The thickness of the endometrium varies with the menstrual cycle during the reproductive age. After ovulation, the endometrium is in the secretory phase under the effect of progesterone, and the thickness of the endometrium reaches 10-12 mm during the 7-8 days after ovulation. This is the change of endometrial thickness during the reproductive age, and the thickness of endometrium of postmenopausal women is within 5mm.