According to the American Heart Association, about eight of every 1,000 babies born in the United States has a congenital heart defect (present at birth) --a problem that occurred as the baby's heart was developing during pregnancy, before the baby is born. Congenital heart defects, also called congenital heart diseases, are the most common birth defects.
A baby's heart begins to develop at conception, but is completely formed by eight weeks into the pregnancy. Congenital heart defects happen during this crucial first eight weeks of the baby's development. Specific steps must take place in order for the heart to form correctly. Often,congenital heart disease is a result of one of these crucial steps not happening at the right time, leaving a hole where a dividing wall should have formed, or a single blood vessel where two ought to be, for example.
Rarely, defects include those in which:
The right or left side of the heart is incompletely formed and is called hypoplastic heart.
Only one ventricle is present.
Both the pulmonary artery and aorta arise from the same ventricle.
The pulmonary artery and aorta arise from the "wrong" ventricles.