With the widespread use of ultrasound, especially transvaginal ultrasound, many women get ultrasound reports with the words “uterine rectal (sunken) fossa” and “pelvic fluid”. At this time, your doctor will explain to you that fluid in the rectal fossa means fluid in the pelvis and fluid in the pelvis means pelvic inflammatory disease. If you accept the doctor’s explanation, a series of antibiotic treatments will follow. When you go for a follow-up ultrasound after “a course of treatment”, the ultrasound report finds that the “fluid” is gone, and the doctor who treated you says that the “pelvic inflammatory disease” is cured. In fact, even if you do not receive “treatment”, the “fluid” will be absorbed by itself in 1-2 weeks because of the absorption function of the peritoneum and the “fluid” will be gone. However, after a short period of time, you will have an ultrasound examination occasionally (for example, when you visit a doctor for menstrual disorder) and you will find “pelvic fluid” again. Naturally, you may wonder: Didn’t you cure pelvic inflammatory disease before? Why do you have pelvic fluid again? Does fluid in the rectal fossa of the uterus mean pelvic inflammatory disease? Why is there fluid in the pelvis? Let’s start with anatomy. The so-called “uterine rectal fossa” is formed by the peritoneum around the anterior wall of the rectum migrating downward in the posterior vaginal vault, and then migrating forward and upward to cover the cervix and posterior wall of the uterus, forming a peritoneal depression between the rectum and the uterus called the rectal uterine (sunken) fossa. ” or “uterine rectal fossa”. Therefore, the utero-rectal fossa is a “fossa” formed by the peritoneum surrounding it. This fossa is at the lowest point of the peritoneal cavity when lying down, which is why it is a “fossa”. The volume of this “fossa” is very small, as long as there is 2-3 ml of fluid, the “fluid” can be detected in the rectal fossa by ultrasound examination. 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 tubes. 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” in the rectal fossa on ultrasound. This “fluid” may be normal abdominal fluid (used as a defense), 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 no need to be overly nervous, and there is no need for immediate “treatment”. The first thing to consider is whether it is: 1. just after the day of ovulation, “fluid” is follicular fluid; 2. just after the menstrual flow, “fluid” is caused by a small amount of backflow of menstrual blood; 3. frequent constipation, the lower abdomen repeatedly at irregular intervals The lower abdomen is repeatedly swollen and painful from time to time, and you want to defecate, but you will not have abdominal pain after defecating. This is because the intestinal peristalsis is not normal and causes the intestines to leak a small amount of intestinal fluid resulting in an increase in abdominal fluid. If you have the above situation, there is no need to rush to use medicine, 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 fluid, the volume of fluid will be more than 100 ml. The pelvic fluid caused by tuberculous peritonitis is the most obvious and a large amount of ascites can be detected in the ultrasound examination. It is worth mentioning: if a woman with infertility who is undergoing ovulation promotion treatment will have more pelvic fluid or even the formation of ascites during the medication, but the doctor who treats you for infertility will determine if and how treatment is needed based on your fluid level. You must follow your doctor’s schedule for ultrasound review.