blue baby, infant born with a congenital heart defect that causes a bluish coloration of the skin as a result of cyanosis (deoxygenated blood). The color is most noticeable around the lips and at the tips of the fingers and toes. The cyanotic condition occurs when a large portion of the venous blood bypasses the lungs. Normally, deoxygenated blood from the veins is pumped from the right side of the heart to the lungs, where it is oxygenated (see circulatory system). In some blue babies, the pulmonary artery is too narrow to allow sufficient blood to pass into the lungs for oxygenation. Surgical correction of the defect is usually required and is usually successful. An incompatibility of fetal and maternal blood types may also cause a bluish coloration in newborn infants, a condition that results when red blood cells in the infant's blood are destroyed by antibodies in the mother's blood (see Rh factor). Sophisticated knowledge of blood types has made this condition increasingly rare.