A dark area of fluid in the rectal fossa of the uterus, or pelvic fluid, is a description of the images seen during ultrasonography. The pelvic fossa is the lowest place in the pelvic cavity of the human body, and when there is fluid in the pelvic cavity for various reasons, it will first accumulate in the pelvic fossa, which may occur for the following reasons: 1. pelvic fluid caused by pelvic inflammatory disease is the most common cause of liquid dark areas in the pelvic fossa. The symptoms are usually accompanied by pain in the lower abdomen and lumbosacral region, fever, fatigue, general discomfort, etc. In chronic pelvic inflammatory disease, fluid in the rectal fossa of the uterus is mainly associated with pain and discomfort in the lower abdomen and lumbosacral region around the time of menstruation and before and after sexual intercourse.2. Ruptured tubal pregnancy and miscarriage can also be seen as a dark area of fluid in the rectal fossa of the uterus during ultrasonography, and patients often have a brief history of menopause and experience sudden onset of severe pain in the lower abdomen and abnormal bleeding in the vagina.3. Endometriosis, accompanied by Symptoms of bleeding during the menstrual cycle and fluid may be present in the rectal fossa of the uterus.