The cause of most congenital heart defects is unknown. Prenatal factors associated with a higher than normal incidence of these disorders include maternal rubella or other viral illnesses during pregnancy, poor prenatal nutrition, maternal alcoholism, maternal age over 40, and diabetes, although it is unclear if any of these actually cause the problem. Transposition of the great vessels is classified as a cyanotic heart defect because the condition results in insufficiently oxygenated blood pumped to the body which leads to cyanosis (a bluish-purple coloration to the skin) and shortness of breath. In transposition of the great vessels, there is no communication between the pulmonary circulation and the systemic circulation. Fresh oxygenated blood from the lungs returns to the heart ready to nourish the body, but instead is whisked right back to the lungs. Conversely, oxygen-poor blood returns from the body to the heart and is then sent right back out to the body without being reoxygenated. 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 rapidly 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. It is the most common cyanotic heart defect identified in the first week of life.
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