There are several different causes of congenital heart defects (CHD) – some are unknown, others are chromosomal, and some are environmental.
In the majority of cases of CHD, the cause is unknown – called "idiopathic". Very often, CHD forms one part of other syndromes of unknown origin, e.g. a child with asplenia may have associated heart defects.
At conception, 23 chromosomes from each parent pair up to form a new human being. These chromosomes contain DNA, our genetic code, the "programme" directing the formation, development and maturation of the baby and all its organs.
In about 3% of babies born, this genetic code has a fault, being either scrambled in places or having a bit duplicated or even left out altogether. A common example of this is Down’s syndrome, in which the child has three instead of two copies of chromosome No. 21. Down’s babies have, among other things, an increased incidence of Fallot's tetralogy, ventricular and atrial septal defects.
This refers specifically to the environment of the unborn baby. Many factors concerning the mother during pregnancy can affect the baby. For example:
If the mother has Rubella (German measles) during the early months of pregnancy, many of the baby’s developing organs can be affected, and s/he may be born with CHD.
Excess alcohol consumption during pregnancy can have the same result.
Babies born to mothers with poorly controlled diabetes also have an increased risk of CHD.
Certain medication taken by the mother during pregnancy can cause malformation in the baby. Epilepsy and acne medication are two such known medications.
Radiation exposure, e.g. if the pregnant mother is exposed to large doses of X-rays in the early stages of pregnancy.
(The Heart and Stroke Foundation SA/Health24, updated January 2008)