Mitral valve prolapse occurs when the valve between the left atrium and left ventricle does not properly close. This valve, called the mitral valve, has two flaps. In mitral valve prolapse one or both of these flaps are enlarged. Mitral valve prolapse sometimes leads to a small amount of blood leaking back into the left atrium, which is a condition called mitral valve regurgitation. Mitral valve regurgitation can sometimes lead to a heart murmur. For the most part, mitral valve prolapse is a fairly common condition and is harmless. Typically, there are no major symptoms with mitral valve prolapse and no treatment is necessary.