Question: What causes Congenital Heart Defects ?
Answer: If your child has a congenital heart defect, you may think you did something wrong during your pregnancy to cause the problem. However, doctors often don't know why congenital heart defects occur. Heredity may play a role in some heart defects. For example, a parent who has a congenital heart defect may be more likely than other people to have a child with the defect. Rarely, more than one child in a family is born with a heart defect. Children who have genetic disorders, such as Down syndrome, often have congenital heart defects. In fact, half of all babies who have Down syndrome have congenital heart defects. Smoking during pregnancy also has been linked to several congenital heart defects, including septal defects. Researchers continue to search for the causes of congenital heart defects.
Source: NHLBI