Blue-baby syndrome, or blue baby is usually caused by a heart defect which laymen often call "a hole in the heart". Normally, oxygenated blood from the lungs is separated from deoxygenated blood from other tissues. Because of a defect in the heart ventricle walls, deoxygenated blood sometimes mixes with the blood from the lungs. The resulting blood going through the aorta has less oxygen than usual. The baby then looks blue. An operation is usually done to fix this. 