Some people have great difficulty emptying their rectum due to what is called rectal descent.
Rectal descent is a problem that appears to be related to childbirth.
When a women gives birth, the normal attachments of the rectum to the lower backbone may get stretched or torn. This tearing allows the rectum to fall into the pelvis where it assumes a horizontal position. The front of the rectum can fall into the top of the anal canal and block the anal opening. In this way the rectal contents can move straight out. When a person pushes to move his bowels, it increases the abdominal pressure.