After ovulation the follicular fluid flows out, a blood body will be formed, the blood body can last for about 72 hours, when the ovarian granulosa cells, follicular membrane cells slowly grow inward to form the corpus luteum, which can be examined in ultrasound. Therefore, the corpus luteum is usually 7-8 days after ovulation, and its volume, size, and function reach a peak, which will be shown on one side of the ovary in the ultrasound, and when you see an uneven or cystic mass, with blood flow signals in the periphery, it suggests that there is a possibility of corpus luteum. The corpus luteum is slowly absorbed prior to menstruation and has to be differentiated from ovarian cysts and abnormal cysts, such as chocolate cysts, which need to be differentiated from cysts formed by endometriosis. If identification is difficult, the patient may be asked to have another ultrasound on the 5th-7th day of the next menstrual period. If the cyst is physiologic, the cyst will slowly disappear, and the presence of a cyst on the previous examination is considered to be a physiologic luteal cyst. If the cyst persists, it is usually necessary to continue the observation a few more times. If it persists after several observations, the specific nature of the disease should be determined based on the ultrasound characteristics and rebound.