The uterorectal fossa is the lowest depression in the body. Seeing a dark area of fluid here suggests the possibility of fluid retention, infection or bleeding. 1. Fluid accumulation: When a woman is in her menstrual period or ovulation period, there will be physiological fluid accumulation due to pelvic congestion, which can disappear on its own. 2. Infection: when a woman suffers from pelvic inflammatory disease or when there is infection in the pelvic organs, a large amount of inflammatory exudate will be secreted under the stimulation of inflammatory factors, and the exudate will accumulate in the uterorectal fossa which is the lowest position, forming a pathological effusion, which is manifested as a liquid dark area during examination. 3. Bleeding: if there is bleeding from the pelvis or abdominal organs, or if there is ectopic pregnancy or rupture of corpus luteum, etc., the blood will flow out and accumulate in the rectal fossa, and appear as liquid dark area. If the examination result shows liquid dark area in the uterorectal fossa, it is necessary to combine the patient’s symptoms and the examination result in order to clarify the cause and then carry out treatment.