The majority of murmurs (extra noises) heard when listening to the heart of a baby are of no long-term consequence. They are the result of the changes which occur when the baby is delivered.
When the foetus is in the womb, the oxygen that the baby needs is supplied by the mother through her placenta and umbilical cord. There is thus no need for the foetus to use his/her lungs and the amount of blood flowing through the lungs blood vessels is small and largely bypassed.
When the baby is born, he/she no longer has an oxygen supply from the mother and now needs to use the lungs to provide oxygen and get rid of carbon dioxide. Blood now has to flow in much larger amounts through the blood vessels supplying the lungs. Because they have been little used during the pregnancy, the tiny blood vessels to the lung may be underdeveloped and blood needs to be forced through them causing turbulence which produces a murmur (compare water flowing through a hose before and after squeezing the outlet – it makes a noise when pinched).
In a few situations a murmur may be caused by disturbance of flow as a result of a narrowing of one of the large arteries taking blood from the heart or a hole in the “curtain” between either the upper or the lower chambers of the heart. More rarely a murmur may be the result of a complex heart abnormality, but in the latter there are almost always other signs present e.g blueness of the baby’s face and lips, breathlessness especially with feeding and/or cold sweats. There are different types of murmurs that can be heard and this will help direct the doctor towards a specific diagnosis.
If a murmur is heard by an experienced Paediatrician, reassurance may be all that is needed, but in most situations a definitive diagnosis needs to be made by an ultrasound examination of the heart (echocardiogram) by a Paediatric Cardiologist.
Hearing of a murmur in most cases should not cause undue anxiety especially if the baby is otherwise well, though it would be sensible to have it checked out with an ultrasound of the heart if there is any doubt about it being entirely benign.