Rectal fossa effusion mainly refers to the effusion in the recto-uterine recess, also called pelvic effusion, which is often found during gynecological ultrasound, or during an MRI. Pelvic fluid can be physiologic or pathologic. When a woman is ovulating, some follicular fluid will flow into the pelvis as the egg is discharged from the follicle, or menstrual blood will flow into the pelvis when menstruation occurs, and the fluid produced at this time is physiologic and usually disappears on its own. There are also pathologic effusions, such as pelvic inflammatory disease, the exudation of inflammation, and the production of pelvic effusion. Pelvic effusion can also occur due to rupture of ectopic pregnancy and intra-abdominal bleeding, and pelvic effusion can also occur due to rupture of ovarian cysts, all of which are pathologic effusions and require treatment.