Fluid dark area can be seen in the tau cavity, in the clinic is often caused by the following reasons.1, intestinal lubricating fluid or follicular fluid after ovulation, flow to the uterorectal fossa, will be seen in the tau cavity of the accumulation of fluid, often the depth of this accumulation of fluid will not be greater than 2cm, belong to the normal phenomenon, and in the clinic will not cause any symptom, do not have to give treatment.2, pelvic inflammatory disease caused by the When there is chronic inflammation in the pelvis, it will lead to oozing, so there will be this kind of tauopelvic effusion. It is often accompanied by pain in the lower abdomen, which is aggravated during sexual intercourse and menstruation.3. Ectopic pregnancy or rupture of the corpus luteum, resulting in internal bleeding in the uterorectal fossa can also be seen in the liquid dark area, so it is necessary to combine with the clinical history of a comprehensive analysis.