Endometrial thickness of 0.75 cm is normal if it is detected in the early secretory phase of menstruation, and it is thin if it is detected in the late secretory phase. The endometrium changes cyclically under the action of the ovaries, and the menstrual cycle is divided into three phases: the proliferative phase, the secretory phase, and the menstrual phase. The proliferative phase is the 5th-14th day of the menstrual cycle, during which the thickness of the endometrium increases from 0.5 mm to 3-5 mm. The secretory phase is the 15th-28th day of the menstrual cycle, corresponding to the luteal phase of the ovarian cycle. By the late secretory phase, which is the 24th-28th day of the menstrual cycle, the endometrium is spongy and can be up to 1 cm thick. the late secretory phase is followed by the menstrual phase, when the exfoliated endometrial fragments along with blood flow out of the vagina, which is the onset of menstruation.