The utero-rectal fossa can also be interpreted as pelvic fluid. The utero-rectal fossa is the lowest point of the female abdominal cavity due to anatomical factors, and most of the fluid leaking from the pelvic and abdominal cavities will flow to it. When fluid accumulation occurs in the rectal fossa, it may be pathological or physiological, so don’t panic. Physiological factors, in the middle of the menstrual cycle, there will be a small amount of fluid flow after the follicles are discharged after the eggs are broken, and there will be fluid in the rectal fossa on ultrasonography. Pathological factors, mostly gynecological inflammation, inflammatory exudate may also be accompanied by abdominal pain, anal swelling and other discomfort, ectopic pregnancy may have uterine rectal fossa fluid, posterior fornix puncture to extract fluid can help determine whether there is ectopic pregnancy. Judging by the appearance of complications of tumor combined with medical history, especially some tumors found late will have effusion present.