What is a heart murmur? What are some common reasons that heart murmurs occur?

Heart Murmur is an abnormal heart sound during heartbeat cycle. Pregnancy and abnormalities of heart valves are common reasons.

Doctors listen to heart sounds by stethoscope. Normally heart sounds like "lub-dup". So there are two sounds : first heart sound (S1) and the second heart sound (S2). Sometimes a water flow like (whooshing or swishing) sound accompanies the "lub-dup". This is called heart murmur. Check out this collection from the University of Washington to hear what the heart sounds like.

Heart murmurs are produced by turbulent blood flow across an abnormal heart valve, septal defect of heart or outflow obstruction from heart; or they may be produced by increased volume or velocity of blood flow through a normal heart valve.

Here is a diagram of heart valves :

Murmurs are produced both in normal and abnormal heart. Heart of pregnant woman and athletes may produce murmurs. These are the examples of "Innocent murmurs".

Other causes of heart murmur are : Valvular heart diseases (mitral stenosis and regurgitation, tricuspid stenosis and regurgitation, aortic stenosis and regurgitation, pulmonary stenosis and regurgitation), Patent ductus arteriosus, ventricular septal defect etc.

Following is a diagram showing murmurs and their causes. Here we can see the red waves of sound in between heart sounds (S1 &S2), these are murmurs.