Nowadays, due to the development of ultrasound technology, ultrasound reports are becoming more and more detailed and comprehensive, and pelvic effusion has also become a common part of the report. Many women get excited and nervous when they see the words “rectal fluid” and “pelvic fluid” on the ultrasound report, thinking that they have a big problem. Is there really a problem? When you consult your doctor, many of them may explain that fluid in the rectal fossa means fluid in the pelvis, and fluid in the pelvis means pelvic inflammatory disease. What follows is a series of treatments with antibiotics. After the treatment, the ultrasound report will show that the “fluid” is gone, but after a while it may reappear again, and then the endless treatment will begin again. Is pelvic fluid an inflammatory condition? Why is there fluid in the pelvis? In the human body, the “utero-rectal fossa” is the lowest point of the peritoneal cavity when lying down, and the fluid in the abdominal cavity will flow into this lowest point when lying down, and the volume of this “fossa” is very small, as long as there is 2-3 ml of fluid, through ultrasound examination, it can be detected in the The presence of “fluid” can be detected in the rectal fossa of the uterus by ultrasound. The peritoneum has absorptive and secretory functions. Under normal conditions, there is a small amount of plasma in the peritoneal cavity to reduce friction between the abdominal organs. If a large amount of fluid is secreted in a pathological state, ascites can occur. The peritoneum also has a defensive function, containing white blood cells and some antibodies in the peritoneal fluid. Because the female abdominal cavity is connected to the outside of the abdomen through the umbilical end of the fallopian tubes, outside pathogens and even tiny particles from the outside may enter the abdominal cavity through the pathway of vagina-cervical-uterine cavity-fallopian tube, and in order to defend against the invasion of outside objects, the female abdominal cavity needs to secrete a certain amount of peritoneal fluid to fight against the invading foreign bodies. On the other hand, because the surface of the ovary is not covered by peritoneum, every month the ovary will ovulate and when the follicle ruptures, it will discharge the follicular fluid into the abdominal cavity and collect in the rectal fossa of the uterus, forming a few milliliters or more than 10 milliliters. Similarly, every month when menstruation occurs, menstrual blood can enter the fallopian tubes from the uterine cavity and “flow backwards” into the abdominal cavity. Therefore, it is generally normal to see a 20-30 mm “fluid collection” in the rectal fossa of the uterus detected by ultrasound. This “fluid” may be normal abdominal (defensive) fluid, follicular fluid after ovulation, or a small amount of menstrual blood flowing backwards. If a small amount of “fluid” (20-30 ml) is detected in the rectal fossa for the first time, there is usually no need to be overly nervous, let alone to “treat” it immediately. We should first analyze it from the following aspects: 1. whether it is in the ovulation period: if it is “fluid”, it may be follicular fluid; 2. whether it is in the menstrual period: “fluid” is caused by a small amount of backflow of menstrual blood; 3. whether it is often constipated: abnormal peristalsis of the intestines may cause constipation. Irregular peristalsis may cause the bowel to leak a small amount of intestinal fluid resulting in increased abdominal fluid. If there is the above, there is no need to rush the medication, you can review the ultrasound after 1-2 months, especially 1-2 months after clearing the stool as soon as possible, to compare. Generally speaking, a small amount of pelvic fluid is not a disease, and a small amount of pelvic fluid can be absorbed on its own. In case of pelvic inflammatory or neoplastic effusion, the amount of effusion will be more than 100 ml. The most obvious pelvic effusion is caused by tuberculous peritonitis, and a large amount of ascites can be detected in the ultrasound examination. It is also important to mention that ultrasound can only report the fluid in the pelvic cavity as pelvic effusion. In addition, when there is intra-abdominal bleeding, what ultrasound can indicate is often pelvic effusion, but if this occurs, the patient often has abdominal pain, abdominal muscle tension and other signs of peritoneal irritation, and a feeling of falling and defecation. When these conditions are present, we should be highly alert and give active treatment to the patient to avoid delaying the patient’s condition.