The cause of most congenital heart defects is unknown. Prenatal factors that are associated with a higher than normal incidence of the disease include maternal rubella or other viral illnesses during pregnancy , poor prenatal nutrition, maternal alcoholism , maternal age over 40, and diabetes . In transposition of the great vessels, there is no communication between the pulmonary circulation and the systemic circulation. There is usually an associated defect that permits the mixing of the systemic and pulmonary circulation to provide some oxygenated blood to the body. Without such a defect, the condition is fatal.
Symptoms appear at birth or very soon afterwards.
The severity of the symptoms depends upon the type of associated defect and the resulting amount of oxygenated blood supplied to the general circulation. The condition affects approximately 40 out of 100,000 infants.