The accumulation of watery fluid within the uterus is hydrometra, and if the fluid is mucinous, it becomes mucometra. The contents in these conditions are acellular or poorly cellular.
In most conditions, the accumulation of fluid is the result of an obstruction to outflow. Cystic endometrial hyperplasia can contribute to this. Obstruction to outflow is seen in improper attempts to remove the uterus during ovariohysterectomy and leaving a portion of uterus behind (see ovarian remnant syndrome). The uterine remnant becomes cystically distended.
Obstruction at the cervix will cause the uterine body and both horns to distend. Endometrial polyps can cause a local obstruction and the uterus will distend proximal to the site. Segmental aplasia and uterine neoplasia are other potential causes. Those cases where obstruction is the cause would be expected to have atrophy of the endometrium due to pressure.
Hydrometra and mucometra are also seen in some cases of cystic endometrial hyperplasia, and varying degrees of dilation may be seen. In mild cases, the dilation is best recognised after transverse sectioning of the uterus (see cystic endometrial hyperplasia above). Marked dilation will require some obstructive situation.