Congenital cardiovascular defects are present in about one percent of live births and are the most frequent congenital malformations in new-borns. In Ireland, there are about 500 – 600 babies born every year with a congenital heart defect. In most cases scientists don't know why they occur. Sometimes a viral infection causes serious problems. German measles (also called rubella) is an example. If a mother contracts German measles during pregnancy, it can interfere with the development of the baby's heart or produce other malformations. Other viral diseases also may produce congenital defects.

Heredity sometimes plays a role in congenital cardiovascular disease. More than one child in a family may have a congenital cardiovascular defect, but this rarely occurs. Certain conditions affecting multiple organs, such as Down's syndrome, can involve the heart, too. Some prescription drugs and over-the-counter medicines, as well as alcohol and "street" drugs, may increase the risk of having a baby with a heart defect.

Other factors that affect the heart's development are under study. The fact is that we don't know what causes most congenital cardiovascular defects.

The word "murmur" literally means "noise". The blood flowing through the heart may make a noise (murmur) which can be heard with a stethoscope. Some of these murmurs are caused by the normal flow of blood (innocent murmurs – see below). Others are caused by abnormal blood flow, for instance through a hole in the heart, or across a narrowed passageway or valve (organic murmur). Doctors may be able to identify a particular type of congenital heart defect simply by the particular sound and location of the murmur. These days however, in addition to a careful clinical examination, the doctor has the benefit of a variety of precise diagnosic equipment, such as echocardiography (ultrasound scanner). This has made the diagnosis of congenital heart defects much simpler and more precise.

Blood flowing through the normal heart and blood vessels does so remarkably quietly in most people. Some of us with perfectly normal hearts have slightly noisier blood flow through the heart and this noise can be heard with a stethoscope. This normal noise is referred to as an innocent murmur. It is heard most often in young children, because they are thin and their circulation is active. The more active the circulation (following exercise or during a fever), the louder the murmur. It is important to emphasise that innocent murmurs, even quite loud ones, are perfectly normal and do not require any particular precautions or follow-up by the cardiologist. The doctor often will be able to identify whether a murmur is innocent (normal) or not simply by listening. If there is doubt, a specialist opinion and investigations (as described above) may be needed.