The cause of most congenital heart defects is unknown. Multiple factors seem to be involved. Prenatal factors associated with higher than normal risk for this condition include maternal rubella or other viral illnesses during pregnancy, poor prenatal nutrition, maternal alcoholism, mother over 40 years old, and diabetes.

There is a higher incidence of tetralogy of Fallot in children with Down syndrome (a common genetic disorder which results from having an extra 21st chromosome).

Tetralogy of Fallot is classified as a cyanotic heart defect because the condition causes insufficiently oxygenated blood to be pumped to the body, which leads to cyanosis (a bluish-purple coloration to the skin) and shortness of breath.

The classic form of Tetralogy includes 4 defects within the heart structures:

• Ventricular septal defect (hole between the right and left ventricles)
• Narrowing of the pulmonic outflow tract (tube that connects the heart with the lungs)
• An aorta (tube that carries oxygenated blood to the body) that arises from both ventricles, rather than exclusively from the left ventricle
• A thickened muscular wall of the right ventricle (right ventricular hypertrophy)

There is flow of deoxygenated (blue) blood into the general body circulation and decreased blood flow to the lungs.

At birth, infants may not show the signs of the cyanosis, but later may develop sudden frightening episodes of bluish skin from crying or feeding (called "Tet spells"). Tetralogy of Fallot occurs in approximately 5 out of 10,000 infants.