Pouch of Douglas (rectouterine pouch): where free pelvic fluid collects
The pouch of Douglas (rectouterine pouch) is a dip in the membrane lining a woman's abdomen and pelvis, between the uterus and rectum, where loose fluid tends to collect.
Remember
When a woman is sitting or standing, loose fluid in the peritoneal cavity (the space inside the membrane lining the abdomen and pelvis) tends to run down by gravity to the pouch of Douglas behind the uterus. Finding fluid in the pouch shows where it has gathered, not what caused it.
The peritoneum is a thin, slippery membrane that lines the abdomen and pelvis. A pouch is a pocket where that membrane folds from one organ to the next. In a woman it passes from the back of the uterus onto the front of the rectum, forming the rectouterine pouch, also called the pouch of Douglas. The name rectouterine gives its boundaries: recto- points to the rectum and uterine to the uterus.
Fluid obeys gravity. Free intraperitoneal fluid (fluid loose in the peritoneal cavity, such as blood, pus or infected fluid) runs towards the lowest part of the cavity, which doctors call the dependent part. When a woman is sitting or standing, the lowest part of the cavity is the pouch of Douglas, so loose fluid tends to settle there.
That is why a scan of a woman with lower abdominal pain looks there. Fluid seen on ultrasound might be blood from a ruptured ectopic pregnancy (a pregnancy that has implanted outside the uterus), pus from infection or ascites (an abnormal build-up of fluid in the peritoneal cavity). The scan shows where the fluid has gathered, not what caused it.
Do not confuse it with the vesicouterine pouch, which lies in front of the uterus, between the bladder and the uterus. In a man there is no uterus, so the equivalent pouch lies between the rectum and the bladder and is called the rectovesical pouch. The paravesical and pararectal spaces lie beside the bladder and the rectum, outside the peritoneal lining (extraperitoneal), so free intraperitoneal fluid does not collect in them.
Key facts
- Position
- the rectouterine pouch lies between the back of the uterus and the front of the rectum.
- Other name
- the pouch of Douglas.
- Why fluid settles there
- it is the lowest point of the female peritoneal cavity when a woman is sitting or standing, so loose fluid runs into it under gravity.
- Front pouch
- the vesicouterine pouch lies between the bladder and the uterus.
- In men
- the equivalent pouch is the rectovesical pouch, between the rectum and the bladder.
- Clinical use
- fluid in the pouch can be seen on ultrasound, and it shows where the fluid is, not what caused it.
Exam traps
- The pouch between the bladder and the uterus is a genuine peritoneal pouch, but free peritoneal fluid settles in the pouch behind the uterus, not the one in front.
- The paravesical space beside the bladder lies outside the peritoneal lining (extraperitoneal), so it is not a place where free fluid within the peritoneal cavity collects.
- A pouch between the rectum and the bladder belongs to men. In a woman the uterus lies between those two organs, so the space is divided into a pouch in front of the uterus and a pouch behind it.
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