Influenced by ovarian hormones, ultrasound can respond to changes in the ovaries and endometrium during the menstrual cycle accordingly. 1. Early follicular stage: the 1st to 7th day of menstruation. There are several small follicles in the ovary with fuzzy borders and slow growth without dominant follicles. Because the menstrual period has just passed, the endometrium is extremely thin and indistinguishable from the uterine cavity line, forming a posterior strong echogenic line of about 5 mm. 2. Mid follicular phase: the 8th to 12th day of menstruation. The echogenicity of the follicles in the ovary increases day by day, the tension increases appearing to have clear borders, and the dominant follicle forms 6 to 7 days before ovulation, reaching about 10mm in diameter. The endometrium is thickening day by day with hypoechogenicity, and the edges of the uterine cavity are strongly echogenic, forming a “three-line sign”. 3.Late follicular phase: Pre-ovulation. On the 13th to 14th day of menstruation, the volume of the ovary on the ovulatory side and the dominant follicle increase day by day, reaching a maximum value until the day before ovulation. The endometrial thickness increases to a maximum value of about 8 to 13 mm on the day before ovulation, with the thickening of the endometrium. 4. Ovulation: Ovulation is an instantaneous process, which is difficult to observe by ultrasound. However, the following signs indicate ovulation: ① disappearance of the dominant follicle; ② a small amount of fluid is seen in the rectal recess of the uterus within 1 to 2 days after ovulation. 5. Luteal phase: the 14th to 21st day of menstruation. The follicles that rupture after ovulation form the corpus luteum with uneven wall thickness and echogenicity. On the 21st day of menstruation, the progesterone level in the body reaches a high level. Hormone testing can reveal the presence of luteal insufficiency.