Well, I keep changing my answer. Here's what I found:
"Medically speaking, the "posterior cul-de-sac' is generally recognized as the area between the rectum and the uterus in the female body. It is also known as the " Douglas Pouch'.The diagnosis of fluid in the posterior cul-de-sac is usually discovered during an ultrasound of the pelvic region, an MRI or a CT scan, or even a standard pelvic exam. If there are abnormalities felt by the physician during the physical exam (which will usually result in some level of pain or discomfort by the patient).This particular diagnosis will also be rendered when the patient had a cyst or cysts that have ruptured. Once these cysts rupture, the fluid that was contained within the cysts is expelled into the cul-de-sac region of the body. Because this particular area is a very common location for endometriosis to thrive, the fluid will frequently be discovered during the diagnosis and treatment of the endometriosis.In the case of the patient for whom the accumulation of fluid is unexplainable or unable to be accurately diagnosed, a procedure known as a "culdotomy' may be performed. This procedure is basically a simple needle puncture and subsequent aspiration or drainage of the fluid within. The fluid is then handed over to the lab for testing purposes. Most commonly discovered is the simple and non-alarming fluid from a ruptured cysts. Not so common, certain components may be found within the test sample of fluid that alerts a physician to a more serious condition or illness, but this is usually not the case. It is very rare that a sample of this fluid will result in anything that is cause for major alarm. There are no tests using this method that are used in the diagnosis of any cancers. More likely than not, this particular issue is temporary and harmless."
http://www.reference.com/motif/health/what-is-fluid-in-po...