Genetic

Association with other disorders-
Fourty percent of neonates with Down syndrome have a form of congenital heart disease, the most common being complete atrioventricular canal defect (Eidem 2000).

Gene Mutations-
Serotonin (5-hydroxytryptamine, 5-HT) is an important regulator of cardiac development. Inactivation of the 5-HT receptor gene has led to ventricular dysplasia caused by impaired myocyte proliferation in mice trials, and probably causes the same effect in humans (Bonn, 2000). Dr. Edward Clark, of the University of Rochester, estimates that up to 50% of certain heart defects are due to flaws in a single gene (Pediatric 1992).

Environment

Pesticides-
The risk of a woman bearing a child with a severe congenital heart defect quintupules after maternal exposure to rodenticides and triples after exposure to herbicides. This information was gathered from the results of a case-controlled study from 1981 to 1989 which showed that 44% of mothers of infants with transposed arteries had been exposed to a pesticide (Althaus 2001).

Maternal Hormones-
Combinations of estrogen and progesterone, such as those in birth control pills, consumed by mothers during the first trimester of their pregnancy is believed to cause some birth defects. However, only a portion of the population may be susceptible to this hormone combination effect because otherwise the numbers of birth defects would be expected to be much higher.

Complexity in Development

The heart is the first organ that forms in vertebrate embryonic development and this formation requires migration, differentiation, and precise interations among multiple embryonic cell types. It is such a precise formation that there is no wonder why and how defects occur (Olson, 1996).