Free fluid is one of the most common findings during pelvic ultrasound. A variety of abdominal and pelvic conditions are associated with free fluid accumulation in the pelvis.
In females, free fluid from the abdominal cavity sinks into the pelvic cavity and settles in the Pouch of Douglas (also called the Posterior Cul-De-Sac and Rectouterine Pouch), located posterior to the uterus and anterior to the rectum. Anechoic fluid in this location can be detected by ultrasound imaging using either the transabdominal or transvaginal approaches.
