A congenital heart defect is any of a number of heart or major blood vessel conditions that are present at birth. They include conditions that either restrict blood flow, allow blood flow in an abnormal pattern, or affect heart rhythm. They are very common, occurring in almost 1% of all births. They are also the leading cause of death from any type of congenital condition.Almost all are genetic, but they can also be caused by environmental factors or obesity in the mother.