A congenital heart defect (CHD) is a heart problem that is present at birth, caused by improper development of the heart during fetal development.

In the majority of the instances when a baby is born with congenital heart disease, there is no known reason for the heart to have formed improperly.

Scientists know that some types of congenital heart defects can be related to an abnormality of an infant's chromosomes (5 percent to 6 percent), single gene defects (3 percent to 5 percent) or environmental factors (2 percent).

In 85 percent to 90 percent of cases, there is no identifiable cause for the heart defect, and it is generally considered to be caused by multifactorial inheritance. Multifactorial inheritance means that "many factors" are involved in causing a birth defect. The factors are usually both genetic and environmental, where a combination of genes from both parents, in addition to unknown environmental factors, produce the trait or condition.