Pelvic effusion, a problem not unfamiliar to everyone, found that there are still some conceptual differences, together with learning ~ ~ according to the principles of anatomy and physiology, the female pelvic cavity, in addition to the reproductive system, there are some visceral organs and blood vessels, lymphatic system. In a normal physiological state, the abdominal cavity will have a small amount of exudate from the internal organs, blood vessels and lymph, about 200 ml, whose main function is to lubricate the role of the organs. It is a delicate internal circulatory system, i.e. exudation-absorption-exudation-absorption, working in a continuous cycle week after week. It is a dynamic balancing system, that is, fluid exchange in the body and fluid exchange in and out of the vascular lymphatic vessels to maintain the normal function of organ activity. On the other hand, after ovulation, as the follicle ruptures, the follicular fluid is ejected and becomes part of the “pelvic fluid”, which is also used as an ultrasound sign of whether ovulation has occurred. After pregnancy, women will also have more exudate accumulation in the pelvic cavity due to the rich filling of blood vessels and the strengthening of various organs. Because the pelvic cavity is located in the lower part of the abdominal cavity and “water flows downward”, fluid will accumulate in the lowest part of the female pelvic cavity, the rectal fossa of the uterus. Depending on the size of the pelvic volume of different people, a “pelvic fluid” of 0 to 3.8 cm will normally be “seen” during an ultrasound. Generally, pelvic fluid less than 3 cm without any clinical manifestations is considered physiological fluid. In the premenstrual period and during ovulation, pelvic fluid may increase to 3-8 cm, which is normal and cannot be diagnosed as pelvic inflammatory disease if there are no special features. In the pathological state, ruptured corpus luteum, pelvic abscess, chocolate cyst, ruptured ectopic pregnancy and cancer of reproductive organs can also cause “pelvic fluid”. The accumulation of excessive free fluid in the abdominal cavity, such as more than 200 ml, is called ascites. The mechanism of ascites is complex and is related to the imbalance of fluid exchange between the body and outside the body and between the fluid exchange inside and outside the blood vessels. However, this is outside the scope of gynecological discussion and belongs to internal medicine. These “pelvic effusions” have nothing to do with pelvic inflammatory disease, so the diagnosis of pelvic inflammatory disease cannot be made on the basis of “pelvic effusions”.