The pouch of Douglas is an extension of the peritoneal cavity located between the back wall of the uterus and the rectum, according to MedicineNet.com. This area normally contains 1 to 3 milliliters of fluid throughout the menstrual cycle, Wikipedia states. This increases to about 4 to 5 milliliters during ovulation. Minimal free fluid is a term radiologists use to describe this finding on a pelvic ultrasound, explains GPonline.
According to GPonline, free fluid in the pouch of Douglas often is the result of a ruptured follicular cyst, which Healthline explains is a harmless, fluid-filled, noncancerous cyst on the ovary. Also called functional cysts, follicular cysts are common in women of reproductive age and rarely cause complications unless they become very large. Free fluid in the pouch of Douglas also is seen following a ruptured ovarian cyst or in pelvic inflammatory disease.
When doctors want a sample of the fluid in the pouch of Douglas, they perform a culdocentesis, a procedure in which doctors insert a needle into the pouch and remove a small amount of fluid. According to MedlinePlus, any fluid removed normally is clear. The doctor may test the fluid for infection or, if blood is present, recommend surgery to determine the cause of the bleeding.