Usually, the endometrium and follicle development are synchronized and both are subject to hormonal cycles secreted by the ovaries. Generally, the endometrium is very thin just after menstruation, often only 0.2-0.3 cm. At the time of ovulation, the endometrium may gradually thicken to 0.5-0.6 cm as the hormone level rises. Then the proliferative phase gradually changes to the secretory phase, and the endometrium continues to thicken and can reach 1 cm in the premenstrual period, or even reach a thickness of more than 1 cm, when menstruation is imminent. The follicles gradually develop along with the thickening of the endometrium, from small follicles to 0.5-0.6 cm of endometrium, reaching 18 mm in diameter during the ovulation phase, to ovulation when they reach 20 mm in length. After ovulation, the hormone level in the body drops rapidly and the follicles begin to shrink, which is the relationship between the two.