The mitral valve annulus may be normal in size or dilated in patients with mitral regurgitation.  The posterior segment of the mitral valve is the thinnest segment of the mitral valve annulus and the most at risk for dilation.  Dilation of the posterior segment of the mitral valve annulus will result in the separation of at least the P2/A2 coaptation zone.  The mitral valve annulus may also be calcified.  Calcification can occur anywhere, but, is more commonly seen in the posterior portion of the mitral valve annulus.  The shape of the mitral valve annulus is oval with its anterior-posterior axis along the A2-P2 diameter usually around 29.1  ± 1.5 mm.  The intercommissural axis of the mitral valve annulus, along the commisures is usually less affected than the anterior-posterior diameter.

The mitral valve annulus is not a rigid ring.  The annulus undergoes changes in size and morphology during the cardiac cycle where the cross sectional area of the mitral annulus decreases during systole and the "saddle shape" of the mitral valve annulus tends to flatten.