A rectocele is the forward ballooning of the anterior rectal wall into the back wall of the vagina, and defecography is the test used to quantify it during straining and evacuation. The radiologist draws the line of the anticipated anterior rectal contour and measures how far the wall bulges beyond it. This depth is the basis for grading.
By the standard scheme, a bulge under 2 cm is small and usually of no clinical consequence, 2 to 4 cm is moderate, and beyond 4 cm is large. The cut-off at which a rectocele is called significant is therefore 2 cm, because at and above this depth patients begin to retain stool in the pouch, strain ineffectively, and resort to manual or vaginal splinting to empty. The larger figures of 5 cm and 7 cm describe big rectoceles but are not the threshold that separates insignificant from significant. The defining limit is two centimetres.
\[\boxed{> 2\ \text{cm}}\]