The pouch of Douglas, also known as the pouch of Douglas, is the lowest part of the peritoneal fold between the uterus (anterior) and the rectum (posterior). It is also the lowest part of the peritoneal cavity of the entire abdomen, and is therefore the first place where free fluid accumulates. Therefore, if an ultrasound detects an abnormality, some people will worry about whether the pouch of Douglas fluid will have any effect?